It's an 1862 Pocket Navy. Actually a 5-1/2' barrel according to specs. Not that rare. Never seen them without the engraving on the cylinder though. And it looks like it may have been worked a little to aid in the placement of the percussion cap. See the groove cut in the relief behind the cylinder? Interesting feature as capping one of those revolvers is the most time consuming part. Does it have the factory Colt patents markings and address?

Keith