I'm probably wrong... but I thought the Richards "Transistion Model" were built on the 1860 Army. The above model looks more like an 1860 frame, but a 1851 barrel fitted and made into a .44. I thought maybe that was just what the Italian clones were modeled after; but, I found this on-line:
The SMALLEST production of 1860 Conversion Models is the Type II, as it bridges the gap between the Richards, and the final change, the Richards-Mason conversions. This Model, has the NEW firing arrangement, where the hammer strikes the cartridge directly through the channel cut in the recoil shield. Whereas the Richards did not, but used a spring loaded firing pin system, and a solid recoil shield. However, this was the LAST conversion to use the PERCUSSION era barrel ! The next full Mason-Richards used the solid frame barrel. serial No.19482X is a superb example of the TYPE II, as few were made, and they saw HARD use! This Mason-Richards conversion has about 75-80% of the original nickel plate, with the heaviest concentration being on the frame and cylinder. The barrel has thinned out somewhat to areas of a light gray, well blending patina. The hammer retains it's case hardening colors, and are vibrantly obvious. The Walnut grips retain about 75% of their original varnish. All factory markings: barrel address( New York America), frame patents( 2 line '72), and all Serial Numbers etc..are crisp, defined, clear and legible. The mechanical action is PERFECT, very crisp and deliberate on each cock of the hammer. The cylinder indexes, and locks tight perfectly, and releases with celerity of fire. This is a very , very fine example, of a very scarce and important conversion for Colt. it represents the steady direction toward the metallic cartridge era, with NO resemblance to the percussion era.
But, it appears I still have a lot to learn. I don't know what the "type I" looks like, but from the above description, the earlier "Richards" model and the "Mason-Richards" conversion are quite different.
It appears the Navy had some of their 1851s converted to .38 Colt:
With the onset of the self contained cartridge era, and the immediate need of cartridge revolvers in the Military, plus the frugality of the Government, it gave birth to the Conversion revolver. ALL Navy Conversion revolvers were originally '51 Navy/Navy percussion revolvers, RETURNED to the Colt factory, and converted to .38 cartridge. They fall in Serial Range 41,000 to 91,000. This Navy Conversion is Serial No. 8951X, and is absolutely correct in every way, a true and perfect Navy/Navy conversion. The Butt and the Barrel bear all the correct markings, as well as a unique feature! the frame was re-stamped with the two line 1872 patent, over top of the original Colts/Patent, and the U.S., remnants of which can still be seen.