The only difference between a true 1861 Navy and the older '51 model is the barrel and rammer assembly. The '61 barrel is styled after the round 1860 Army, but 7 1/2" long (like the '51 Navy).
Some reproductions are now being made that use the 8" 1860 length barrel, but bored for .36 revolvers.
Besides what's described above, the 1860 used a modified Navy frame that could hold a .44 cylinder, and had a slightly longer grip. Otherwise the 1851 Navy, 1861 Navy and 1860 Army use the same parts.