It does appear to be a collar disc, but that attachment device confuses me. During WWI there were some devices like yours that actually fit on a leather strap. That may be the answer. Try "US Army Collar Disc" in your search engine and you will see bunches of those.
It seems that would be off of a Model 1876 scabbard: 45 / 70 Long Loop Scabbard
I don't know which other models may have used these, but this scabbard model is described as rare.