Many years ago, in the days when steam locomotives had slide valves and slide valve steam engines were in common use, the matter of getting the steam chest port faces flat was a regular event. In locomotive steam chests, the steam chest was cast integral with the cylinder, and vertical milling machines, let alone of a size and type to handle a steam chest face did not exist. The usual manner was to hand chip the rough cast face to approximate flatness and near final dimension relative to the centerline of the cylinder bore. This was done using a cape chisel and hammer. Following the chipping-off, a very coarse bastard cut file, sometimes bent to make it work inside the chest- was used to take the chipped surface down to closer to flat and checking for parallel with the centerline of the cylinder. A series of files, some hot-bent in the forge and rehardened/retempered were used. When the steam chest face was as good as could be gotten with smooth cut files, the machinist then hand-scraped the face to flatness. A final lapping might be done using a small lapping plate.
In the OP's pictures, the Stanley Steamer steam chest presents quite a puzzler. Thinking back to the times when that engine was built and what machine tools were available is a start. The obvious, but wrong answer, is to suggest a slotter or vertical shaper was used to machine that steam chest face. As was noted in this thread, the tool marks when machining something like a steam chest face should run parallel with the direction of travel of the valve. My guess is the Stanley Brothers shop used the old methods of hand chipping, finishing with files that might be hot-bent for the work, and then hand scraping and final lapping.
A slotter would need 'runoff room' for the tool to travel beyond the bottom edge of the steam chest surface. It would also put tool marks perpendicular to the line of travel of the valve. It might be that a slotter, or perhaps a setup on a shaper, was used to rough machine the surface of the steam chest. Ultimately, my belief is that it came down to hand finishing. Of note are the steam and exhaust ports on that surface. In order to work properly, the edges of the ports need to be straight and sharp cornered. Similarly, the slide valve surface has to also have straight and sharp edges and corners on the outer ends and inner 'pocket' ends to cutoff steam and prevent leakage under the valve and resulting steam cutting. Hand scraping of the steam chest face, finishing the port slot edges with files, and then lapping the steam chest face seem the most likely way that engine was built.
With today's small air die grinders having 90 degree heads and some small diameter abrasive discs, it might be possible to 'rough down' the surface of the steam chest to something close to flatness. Using a straight edge and feelers and possibly a small flashlight, along with the die grinder, roughing down to get rid of the bulk of the cast iron could be done. Once that was done, it would be time to fall back on files, hand scraping and final lapping. A small lapping plate on a steel flat bar handle might be used to lap-in the steam chest surface.
With no disrespect to SIP6A, I take exception to some of the points he stated:
As for 'wringing together' and galling: steam chest faces and valves were made as flat as possible to avoid steam getting between them. Slide valve engines ran on saturated steam as a rule. As such, there was always some condensation present (this gets to a property known as 'steam quality', a determination of how much moisture the steam has 'along for the ride'). In addition, there was often steam cylinder oil carried along by the steam. These lubricated the sliding surfaces. I've seen plenty of steam engines with cast iron slide valves and cast iron steam chest faces that have run for ages without any galling or other damage. The slide valve has to float on the valve stem in the direction perpendicular to the steam chest face. This lets the steam pressure in the chest push the valve hard against the chest face, sealing the ports.
Getting surfaces so flat as to 'wring together' is in the league of gauge (aka 'Jo') blocks or similar. Not done and not likely to happen with a steam engine's slide valve and steam chest face. Another thing that was done was to make the slide valves out of bronze, or at least face them with bronze to improve wearing properties and reduce friction. Old slide valves that were badly worn were sometimes built up using oxyacetylene brazing and then remachined, giving good service.
On some of the slide valve steam locomotives, wear of the steam chest faces was a real issue with the engines in constant heavy train service. Pulling down a locomotive to the point of being able to get the cylinders onto a planer to recut the steam chest faces was the last resort and only done at a major shopping. Some railroads solved the problem by using a replaceable seating surface in the steam chests. This was a cast iron plate with both faces planed off parallel and flat, then hand scraped. Some railroads used studs with a countersunk head and a breakaway shank to secure this plate to the steam chest face. The breakaway shanks left the stud's head seated in the countersunk area, below the surface and out of the area of steam paths. A soft copper gasket was often used between this plate and the steam chest face. After some time, the realization was reached that the same steam pressure which seated the slide valve could be used to clamp this plate in place. Stops or dowels were then used to hold the plate in location in the steam chest, and the pressure did the rest.
Again, turning our minds back to the era of the Stanley Brothers and what was available, as well as what shop skills were absolutely required: chipping off the faces with a hammer and cape chisel and hand filing and hand scraping would have been as normal and common a thing as expecting to find a Bridgeport in most shops today.
As I wrote, we live in a wonderful age with all sorts of handy little powered tools like air die grinders and a veritable arsenal of abrasive discs and other finishing products. Years ago, we had some heavy damage happen to the thrust bearing 'tub' on an old hydro turbine. The bottom surface of the tub was originally machined flat on a vertical boring mill. We did not want to send that part of the generator out for remachining as it would have required some tricky rigging and been a way over-wide load. I opted to have the crew re-face the bottom of the thrust tub in place. We used some precision straight edges and levels to determine the 'map' of the bottom of that thrust tub. The rotor shaft of the generator extended up thru the center of the bottom of the thrust tub, making most in-place machining ideas pretty nigh impossible at the time. We used 4 1/2" angle grinders with abrasive discs to rough in the surface to something resembling flat, and kept checking with the precision straight edges and feelers, moving to air die grinders with 2" diameter discs and ultimately to hand scraping. No fancy scraping, just what we'd do when scraping in flat surfaces on thrust bearings. We were not going for a true sealing surface, just good bearing contact with the bottom of the thrust bearing assembly which was dowelled to the thrust bearing tub bottom. What had happened was a loss of oil in the thrust tub and operator error combined. The bearing was run dry, the babbitt melted out of it, and the rotating collar on the shaft galled to the cast iron thrust plate that had formerly been babbitted. This galling was so complete it amounted to a friction welding job. The result was the entire thrust bearing carrier, which was formerly dowelled into the thrust tub bottom surface, became a giant fly cutter with the sheared off 1 1/4" diameter dowel pins as the toolbits. The thrust tub was cast integrally with a large 'bearing bracket' ( a star shaped casting about 16 feet in diameter) and was made of some 1921 GE mystery metal, probably a cast semi steel as near as we could tell from the chips. We spent a few weeks on the hand resurfacing of the bottom of that thrust tub, and when done, I could put a Starrett 98 level on the surface in all directions and be pretty much dead level. We used precisions cast iron straight edges made by Busch Precision for the job.
Another tidbit on this same vein: years ago, we sent some hydro turbine machine work up to a machine shop in Port Colborne, Ontario. The then-owner was a fellow who had started out in life as a merchant marine engineer in Holland, sailing with recip steam main engines and steam driven auxiliaries such as windlass engines, pumps, etc. He had emigrated to Canada in the 1950's. At some point, he got work during the winter months when 'Lakers' and smaller ships running in the Welland Canal were laid up, doing machine work to overhaul engines, pumps and the like. He realized that a lot of time and money could be saved if steam chest faces on steam pumps and other engines could be re-surfaced in place aboard the ships. The result was he took parts off an old travelling head shaper and made a portable shaper that he could setup right on a steam chest. This was driven by an electric motor, and was easy enough to hoist aboard a ship and setup on a steam chest.
The Stanley Steamer engine in this thread really caught my eye. I have never seen a Stanley Steamer engine in detail like this. It is as if the Stanley Brothers were going out of their way to dream up something that was nigh-unto-impossible to machine, even with what we have available to us in this day and age. I can appreciate the Stanley Brothers wanting to make things as compact and light as possible in their engines, but the steam chest design is taking it to extremes. Just getting the fine-threaded plugs that act as 'steam chest covers' out after seeing steam service and the passage of time and rusting could be a challenge. I know the oldtimers used a paste of graphite and steam cylinder oil as the fore-runner of what we know as 'Never Seez' or anti-seizing paste/lubricant. Seeing those fine threads, all I could think of was having to heat things up with a torch, soak in the penetrating oil, try to break that plug loose and keep trying... knowing if things went wrong there would be some stripped threads or a broken and irreplaceable casting. A 'normal' steam chest would have had a flat cover with studbolts to hold it on, but this engine was too compact and too lightly built for that. A lot to ruminate on, seeing the photos.
The other side is only worn about .002" 
