I would have thought that the U.S.Quartermaster would have stamped U.S. on anything and everythig that came to the Ownersip of any U.S.Regiment. However I am more a shooter than a Historian. Both Hawg, and Captain Kirk would be better qualified to answer that question for you.
Congratulations on finding an original though. I have never gone into collecting the originals, because I feel they rightly belong to those persons who either have a Collectors Frame of mind, and wish to preserve for later generation the original weapons that their Forebears used, or who are curators of Museums, and can retain them in good shape, so that future generations can see the, "Real Thing!"
I have Replica Remingtons, and since the design is so very reliable, and I am an "Old West Afficiondo," I even carry them for Self Defense. I know that if I happen to lose one, (God Forbid!) I shall only be out of pocket to the extent of just over $200.oo, and although that would be a severe blow to me in my, "Fixed Income Condition," It would not be insurmountable, and some scrimping and saving for a couple of months, could conceivably relace the missing Pistol!
However I am sure that both Hawg, and Captain Kirk will be either able to answer yout question, or refer you to someone beter historically quaified that myself. There is a saying on this forum, that if there is no picture, it never happened. So some photo's of your recently acquired treasure would be most apreciated , especially by Hawg and Captain
from here in Sunny Arizona!