Someone did indeed cut down an M1861, looking at it though I don’t think it was a period job. I can’t tell you why but I’ll wager there are some repro parts in the mix.
That CS mark on the barrel is spurious and the Patina of the barrel is off in relation to the rest of the arm. I’m tempted to say someone decided to bubba a Bannerman. I hate to say it but I would have avoided the piece.