Muskets were used until '68. Easy to tell as the pre-68s didn't have serial numbers. The '68 50-70 TDs did.
They were produced until the Model 73 Trapdoor was developed.
The bottom gun in this photo is the '68 version of the 50-70 trapdoor, the top is the a 1878 made Model 73.
The 1866 Springfield was know as the Second Allen Conversion, and it was used in the Wagon Box and Hayfield Battles, both in 1867 and gave good accounts of themselves.
Its doubtful any 50-70 Trapdoors were used by the Army at Little Big Horn.
If one is interested in what rife was used where, the should contact Casey at the Wyoming State Crime Lab, Firearms Investigation.
For her college Theses, she did a study on all the firearms and recovered bullets she could find among the museums in the Mountain West, tracing bullets to different battle fields (bullets from one rifle she found she traced to 4 different battles ending up at the Little Big Horn. She told me it was from an Indians' rifle so I'm going to assume it was a 50 cal bullet. I'll ask her next time she comes back to town.
She was also going to give me a copy of her thesis, and with her permission I'll post more info.
She owes me, it was her mother, a county commissioner, who got me stuck doing Women's Firearm SD classes which has turned into 5 years of nothing but a headache.
She owes me.
Any way, this young lady is quite knowledgeable of the period weapons of the Indian wars of the Mountain West.