Search for "Bannermann". The Bannermann company seems to have bought up all the spare parts stocks when the Trapdoor was taken out of service by the US Army. So if the parts all look original, but the mix of parts is not quite right, then the chances are it could be a Bannermann "bitsa".
Get hold of "The 45-70 Springfield" by Frasca & Hill. If it's not in there, then it's a homebrew gun.
Patrick