A 51 is a .36 caliber handgun with an octagon barrel and a hinged loading lever. It also has the so-called "Navy" grip which is basically the same as that on the Single Action Army (the most popular handgun grip ever designed). The 60 is a .44 caliber with a round barrel and a streamlined creeping loading lever. It has the over-sized, longer "Army" grip. The 1860 was the only Colt revolver with this large grip. Some of the Italian gun markers simply mix and match parts to come up with guns that never actually existed. So, you can find what they call a .44 "Navy" which is the 60 frame and cylinder with Navy style grips and an octagon barrel bored out to .44. You will also find other combinations of caliber and grip styles called all kinds of names that never historically existed. Heck, Cabellas even has "histories" of these non-existent guns on their web site. Sort like all those Hollywood movies "based on" or "inspired by" a "true story" that never took place.