Also read somewhere that the US changed from "Company" to "Troop" in either 1881 or 1883 so if true, it wouldn't have been stamped until then or afterward. (from the original post)
Then I'm reading in Roy Marcot's book a copy of letter written by a J. R. Dunkelberger, Lt. Col. 1st Cavalry sent to Chief of Ordanance in Washington DC on April 14, 1870 from Headquaters Troop - 1st Cavalry, Camp Grant.
On that same page there's another letter sent from Headquarters, Troop C, 3rd US Cavalry, Fort Bascom, July 30th, 1869 from F. Van Vliet, Captain, 3rd US Cavalry, Bvt. Lt Colonel USA to M.S.G. of Ordinance at Fort Union.
My point is it seems they were maybe using the word "Troop" instead of "Company" a dozen or more years before it was "officially" used by the US government. I'm guessing there are other examples of same elsewhere.
If you want a good laugh I can post the letter from Dunkelberger (as long as I'm not breaking copyright rules). He has nothing good to say about Spencers. Twogunpete