Bayonets of the world is my first check always. Barring that I go to my jantzen book. It's got the exact measures of an 1861 but is just too big for the barrel
The first thing you need to look at is the socket size. The interior diameter at the bore end will tell you a lot. There is a substantial difference in that of a 1861 and 1842. And it could even be for something else.