John,
Serial numbers are well and good and usually tell the tale on Colt revolvers. You obviously have some background in Colt firearms history. All said it has a 5-1/2" barrel, has all the original Colt numbers and "autograph", but has what appears to be a questionable cylinder. An 1851 Navy has a cylinder that is straight from front to back like this revolver. No step down like a "pocket" model does; a decrease in diameter from the face of the cylinder to the rear of the cylinder. The frame having a cut out accepting this same configuration of differing diameters. In addition 1851's and 1862's have a bead front sight not a ramp like this gun has. SO.........what is it? The barrel configuration says 1862 except the ramp site. The cylinder configuration says 1851 except the ramp site. Did you notice, as I did, the groove cut in the frame relief that might aid in positioning the caps? Upon further research and more time looking at the picture, I think this gun was a customized (aka: butchered) 1851 Navy as the serial numbers would attest. Did any Navies come with ramp front sights? I haven't found any info stating so on any reputable website or in any weapons books that I have. I "shot from the hip" on this one as the barrel was easily explainable, minus the ramp site. What do you think this guy has? It's obviously not "off the shelf".

Keith