Hellgate, the hammer spur came down on the New Model Army, which came out in 1863, and utilized the Beals patent of 1858, which is why the New Model is commonly referred to as the model "1858"........replicas are New Model Army's, the 1861 didn't use the beals patent for retaining the base pin, it can be pulled forward with the rammer in place under the barrel.....this apparently was not a really great thing, as with wear, there's not really anything to keep the cylinder pin in place. Mine, as well as others I have seen, have a field modification, there is a screw set into the rammer to hold the cylinder pin in place.
Also the profile of the hammer is like the Whitney and Beals revolvers that preceeded it, they had high upswept spurs......it's really nice when you use it.
From what I've read, there was about 2000 1861's made.