We've discussed old boilers and old pressure vessels many times on this 'board. A small vertical fire tube boiler is simply an old boiler with an unknown history at this point. The UT will tell you a lot. Before you get any further with that boiler, I would check your local and state boiler codes and regulations as to whether you will be allowed to put that boiler into steam service. Granted, it is a historic old boiler. However, it does have a lapped longitudinal seam on the barrel. Lapped seams are prohibited in most states. The lapped seam raises a red flag for me. Before you invest time and money into the boiler, be sure that- assuming it were otherwise sound- it could legally be put into steam service with that lapped seam.
If you find there is no code or legal issues with the lap seam, then the next step is to do a thorough visual inspection on the boiler. With today's fiberoptics and miniature video cameras, a person can get a look inside of parts of boilers that previously required cutting an inspection opening. Your boiler is simple enough so that by looking through the tube holes, you should be able to see most all of the internal surfaces. If nothing jumps out at you (such as an area that is visibly wasted or thinned, or cracks in the flue sheets or excessivley wasted rivet heads), then the UT is done. Once you have the UT in hand, it is a matter of interpreting it. The "beam" from a UT transducer can often be reflected back when it finds "laminations" (slag inclusions in the steel plate, there since the plate was rolled at the mill). A lamination will give a false reading, showing a much thinner spot. Those laminations usually never hurt anything, and prior to UT, no one knew they were there. Interpreting the UT so as to recognize anomalies vs legitimately thin or pitted areas takes a little doing.
Once you have the UT in hand, and determine the thinnest areas of each of the boiler sheets, you can then run a set of calculations on the boiler. Based on as-found dimensions, including riveted seam layouts, the "minimum thickness required" for a given maximum allowable working pressure is determined. When you have an old boiler such as this one, I would simply take the thinnest readings on each sheet of the boiler and run a calculation to determine what the maximum allowable working pressure is. Use the tensile stress value stamped on the boiler, and use a minimum factor of safety = 5.
I would suggest getting in touch with the shop at the California State RR Museum in Sacramento. I had a contact there name Dennis Daugherty, for whom I did some engineering relating to various steam locomotives. They have a good sized back shop. 2" diameter fire tubes were commonly used in locomotive boilers. Possibly, they will have some extra 2" tube material and be able to loan/rent you a tube roller. Another place would be the Big Trees and Roaring Camp RR in Santa Cruz, or the Skunk Train up in Fort Bragg. I did some engineering for the Skunk Train years ago as they had a crack in the boiler backhead on a Baldwin steam locomotive. I designed the repair and submitted the Form 19 (Alteration and Repair Report) to the Federal RR Administration. Any of these historic railroads should have tube rollers for 2" tubes and possibly some extra or cutoff 2" tube material.
Another thought: on firetube boilers with hard firing such as on locomotives and some Scotch Marine Boilers, it was common practice to "bead" the ends of the firetubes at the entry of firebox end. Beading consisted of flaring the tube ends and then running around them with a "bootleg" shaped tool in a riveting or chipping gun. The bootleg tool doubled over the tube wall, kind of like rolling up the cuffs on the legs of jeans. The doubled-over tube wall was driven down so it met the tube sheet. A seal weld was then run around the doubled over tube end, and a final rolling was done to insure no leakage. The seal weld was to prevent hot gasses and cinders from cutting or eroding the end of the tube where it projected from the tube sheet, and the doubled-over end and slight flare made a much smoother entry for hot flue gas which often contained fine cinders (or sand if on an oil fired boiler, where "sanding out the tubes" was often done to get rid of carbon soot. For your little boiler, my own druthers on this are to roll in the tubes, and try to get a roller which also flares the ends. Just that much more insurance that the tubes are well set in the sheets, and a much better entry for the flue gasses. Flaring up at the smokebox end also makes sliding in the turbulators that much easier.
Word to the wise: You do not know how many times that boiler has had new fire tubes rolled in, or how many times the fire tubes had to be re-rolled to deal with leaks. The result is the tube holes in the tube sheets can often be considerably larger than original diameter. When this happens, rolling in the new tubes can result in excessive rolling to expand them to meet the enlarged hole in the sheet, with some localized thinning of the tube walls and possible cracking or splitting. What the boilermakers did in this sort of situation was to use copper ferrules on the tubes. The ferrules took up for the oversized holes in the sheet. The other plus side to using ferrules is they are a bit more forgiving as far as keeping the tubes leak-tight in the sheets. Some firetube boilers were ferruled from the git-go, with one sheet figured as the "fixed" sheet (usually the sheet at the smokebox end of the barrel), and the other sheet (usually the firebox tube sheet) as being the more likely sheet to flex a bit. The fire tubes were ferruled in the firebox tube sheet from the git go on a lot of locomotive boilers.
Your boiler has no wet leg nor submerged upper tube sheet. It is as simple a boiler as it can possibly be. My own concern is the fact that the longitudinal seam is made as a lap seam rather than a butt joint. Lap joints are prohibited in most states, even on historic boilers. The reason for this is the lap seam puts the rivets in the seam into a combined loading condition: shear plus bending. Inspecting that seam is a whole other matter. UT will tell you how much of the boiler barrel sheet is left. Sounding each rivet with a hammer and inspecting rivet heads inside the boiler for wastage (reduction in material) is a start. The seam is a very high stress area, and you have a small old boiler with an unknown history. Whether water treatment was ever used is anyone's guess, so I'd be looking for signs of oxygen pitting and loss of material down where the bottom flue sheet meets the barrel from corrosion/mud accumulation.
The type of boiler you have was often used in stores which did steam pressing of clothing, usually in conjunction with tailoring and dry cleaning. These were basic vertical firetube boilers with no wet leg nor submerged upper tube sheet. Steam was a once-through proposition as it was used to run/heat a clothes pressing machine or used to steam the clothes to take out wrinkles. Another place where this sort of small boiler might have seen use would have been in a small creamery.
The ultrasonic thickness testing is a start. A visual inspection made by a competent person such as a boilermaker used to working on riveted firetube boilers is a must.
The pipe plug in the middle of the tube sheet raises another flag. If that plug is in the lower (firebox) tube sheet, it might be plugging a hole formerly used for a fusible plug. A vertical firetube boiler of this type usually came with a "dry" firebox/base. Tailors and cleaners used these boilers in storefront type operations, and usually fired them on gas from the mains (if in a city or town with gas supply). If the boiler was fired with coal, then a base with firebrick lining, grates and ashpit was used. Up on top, there was usually a conical "smoke hood", which fitted onto the barrel and reduced to whatever size smoke pipe was used. On this type of boiler, the hot flue gasses tend to rush right through the firetubes and a lot of heat is lost up the stack. To address this and try to capture more of the heat from the burning fuel, "turbulators" were often used on this type of boiler. These are strips of thin-gauge sheet steel, just wide enough to slide into the fire tubes, and cold twisted. The turbulators are inserted in each firetube from the top flue sheet, and have tabs to keep them from falling thru the tubes and into the firebox. Turbulators are easily made, and being thin sheet steel, are probably long done to rust or removed and thrown in the scrap when the boiler was removed from its installation.
My guess is this boiler was probably like the proverbial red-headed step child. It was in some installation where no one was a real stationary or marine engineer, and they just fired it to make steam for some process or equipment (such as a clothing press). It got minimal maintenance, likely little or no water treatment, and may have been steamed at some much lower pressure than the original MAWP (maximum allowable working pressure) of 100 psig.
You have to start at "square one" with this boiler, and you will need to determine if you will be allowed to steam a lapped-seam boiler. If you clear that hurdle, then you move to doing a thorough inspection, UT, and full set of calculations before you start chasing down new tubes and a tube roller.