Haymaker,
It was great to meet you at the Muster, and I look forward to shooting with you again next year.
Just off the top of my head, but mostly to bring up some ideas that could be greatly illuminated by our more learned colleagues:
I am aware that certain regular Army personnel having special skills were designated as "artificers" (sp?) These included men skilled as carpenters, blacksmiths and foundry workers, mechanics, machinists, wainwrights (wagon makers) and masons, even electricians, later on. The Army did not always rely on "civilian contractors" to maintain equipment , although I would think it more expedient to do so on the Frontier. These specialists would probably have been posted at the larger, headquarters-type forts, depots or arsenals, like St. Louis, or The Presidio, possibly Ft Leavenworth, Ft Sill, or Ft. Riley. I would think that any frontier post would have at least a Saddler and Blacksmith assigned. The Saddler would maintain or make all leather equipment, including uniform belts and holsters. The Blacksmith would of course shoe horses, as well as some mechancal repairs (wagons, door hinges, buckles, etc.) even minor small arms repairs (springs, hammers, firing pins). Often worn or damaged equipment, such as artillery pieces, or other mechanical equipment was expensive to replace, and had to be repaired or refurbished to continue to be used. Remember, after the Civil War, Congress severely cut spending for the Army and other services - C.W. uniforms (and even food!) were still being issued for years afterward. Many things had to be cobbled along until they were completely worn out.
The Corps of Engineers were the builders of forts, bridges, gun emplacements, trenches, levees and other major construction. They also performed surveying and the laying out of roads, and included Sappers and Miners. Robert E. Lee designed and built several forts on the East Coast before the War. The US Military Academy at West Point is, in fact, a School of Engineering, and most of the top graduating officers went into the Engineers. I never hear much, if any, about enlisted men in the Corps of Engineers, though. I don't know if the enlisted had any sort of distinctive insignia or uniform trim, other than the "castle" cap badge. Most enlisted engineering involved the use of a shovel anyway, so who needs a specialist?
Ordnance personnel had uniforms trimmed in a dark red, almost maroon material (I think it was termed "Crimson", as apposed to the Artillery "Scarlet"). They did major maintenance on artillery and ammunition, as well as the carriages and caissons. I believe they weren't assigned any lower than Division level, so they probably would have been posted at places like arsenals and depots, but occasionally, I could understand them being sent "to the field", in certain circumstances. They would have included wheel- and wainwrights, mechanics, machinists, and foundry workers (patternmakers and molders)
Each of these mentioned above had at least a "distinguishing mark" of some kind: I believe "artificers" had crossed hammers. A soldier fully kitted out in regulation gear would be instantly recognizable for what he was. Of course, a "field uniform" would have consisted of a mix of various issues of uniforms as well as civilian attire thrown in as needed. As stated above, the supply system was woefully understaffed and underbudgeted, so the soldiers out on the Frontier had to do with was on hand, for the most part.
Just my 2 cents, I hope someone more knowledgeable can straighten me out, if I'm off in left field on this.