The statement in question refers to relics found at a Plains Indian wars battle site of the 1870s:
The fact that the majority of the Indian cartridges are Spencers [.56/56; .56/52; .56/50] that were not manufactured after 1866 suggests that they were conserving the cartridges they had and even going to great length to curate their ammunition.
It could be read that the author was referring to the Spencer firearm as not being manufactured after 1866. If he’s referring to the firearms, it could be considered correct, but if he’s referring to the cartridges – which I believe he is – it would be incorrect.
Roy M. Marcots’s work, Spencer Repeating Firearms, makes it clear that the three Spencer cartridges mentioned above were manufactured and made available for many years after 1866. The Union Metallic Cartridge Company began manufacturing the .56/56 in 1868, and Winchester Repeating Arms took up production of the .56/56 cartridge in 1874. The United States Cartridge Company began producing cartridges for all caliber of Spencer rifles, including the .56/56, .56/50, and .56/52, in 1869 and continued offering these cartridges until 1908.1
- Marcot, Roy M.Spencer Repeating Firearms. Rochester, NY: Rowe Publications, 1990. 213, 215, 216.