Mine is #1021 and it has the round nut holding the mainsprings in place. Mine has no cartouche. If yours has a cartouche it was most likely military issued (and probably not a prototype). I did some research when I first got it and many of them were issued to sailors on the U.S.S Constitution during the Civil War (it was a training ship then).
I think the info I got was from a library book on military weapons, or Navy weapons. I'll check.