Adam:
The boiler was built by Eclipse "Lookout", a well known builder of vertical firetube boilers. You are correct in that the boiler was originally built for solid fuel firing, and someone added an additional dry firebox ring for the oil burner. As you also note, the boiler has a unique design in terms of the way the smokebox (top) end is designed. To retube the boiler, it would be a real job. My guess is retubing might well involve busting rivets and knocking them out to remove the top head and then cutting thru the top sheet of the smokebox. The smokebox is 'wet' (submerged) from the looks of things, with the smokepipe passing thru the water space and out the side of the barrel. I've never seen this design of VFT. It looks like a boiler designed for a tight location, which would be about the only reason anyone would order such a boiler.
My guess is the boiler may well have been ordered for use in a small installation such as a small steam laundry, or dry cleaning establishment and used for steam pressing. Or, the boiler may have been ordered for use in some small business needing a small amount of steam for processing such as sterilizing or cooking/canning.
I am as much at a loss as you are as to why anyone would order a boiler that would be next to impossible to maintain, let alone retube. Even 'punching flues' to clear them of soot deposits is next to impossible with this configuration of boiler. Eclipse Lookout built VFT boilers to ASME code, and this boiler, having the handholes and design features that it does, may well have been built to an earlier ASME code edition.
As you note, drafting on this configuration of boiler was going to be another matter. The traditional conical 'smoke hood' over the smokebox, with a straight shot up the stack gave much better- and 'freer' drafting. This design is convoluted and would require a tall chimney or stack to get any kind of good natural drafting. As originally built, for solid fuel firing, the customer likely had to have had plenty of tall stack. A small boiler stuck in the basement of a shop with 3 or 4 stories above it would have had plenty of chimney height to get some kind of drafting. Taking the line of thinking that the boiler was ordered for a small business in a city and stuck in a basement setting, the side-outlet smokebox would make sense. This would give maximum height to the barrel/steam space while keeping within headroom limits in a basement.
Every so often, in driving around various areas, I will see a 'tall stack' on a what had been a small manufacturing or food processing plant. These businesses are usually long gone, but the stacks remain. These are sometimes brick masonry or sometimes riveted steel, maybe 30-40 feet tall at most. In Kingston, there was a small local meat processer who put up sausages and cold cuts. The stack is maybe 18" diameter x 35 feet tall, still in place. Years ago, down in Brooklyn, I'd see very small VFT's installed in storefront dry cleaners for running clothes pressing equipment. These were riveted construction, usually fired on gas, and used city water main pressure (about 40-45 psig in Brooklyn) for feedwater supply. I don't think they carried much more than 10-15 psig for steam pressure, so qualified as 'low pressure' boilers. The food processors may have carried higher pressures, needing higher temperatures for cooking, pasteurizing, or sterilizing.
A U/T along with a mirror and good light would tell a lot about that boiler. Without being able to get to the top tube sheet and submerged smokebox, the boiler is a real potential can of worms. The smokebox is fully submerged in this design, given the safety valves tapped into the top head. That design puts anyone wanting to really inspect and investigate the boiler in the blind. My guess is that if a person wanted to use this boiler, they'd wind up removing the top head and top sheet of the smokebox to inspect and retube it. Now for the real zinger: imagine building that boiler in the boiler shop. My guess is the 'guts' of the boiler were made up as a unit, i.e.- bottom wet firebox with bottom tubesheet, top submerged smokebox minus its top sheet, and tubes were rolled in and beaded (at least on the firebox end).
This created a 'gut package' and the top sheet of the submerged smokebox was riveted in last. A boilermaker reaching thru the smokepipe opening could handle a rivet snap and some sort of bucking bar. When the 'gut package' was fully assembled, it was then slid into the barrel and tied in at the mudring. A shop hydro test would disclose any leaky tubes, and again, a boilermaker with a long arm reaching in the smoke pipe hole could handle touch up rolling.
It is not what I would call a practical or maintainable boiler. With an unknown past history, lapped barrel seam, and the smokebox design that it has, I'd pass on it even if it were in close reach of my home. I have friends with heavy equipment and the means to go get a boiler like that one and haul it home and set it. We worked on a few locomotive boilers along the way and we have plenty of boilermakers' tools, aside from means of moving and hauling the boiler, but this is one boiler I know we'd all steer clear of. Call it a can of worms- literally.