Another special post, because he has a lot of good information.
This post is specifically a question on the old blackpowder arms.
As near as I can tell from the literature, the difference in designation between the "Army" and "Navy" names on the mid-19th century Colt's revolvers is in the first, or perhaps primary, caliber in which they were manufactured.
Specifically, the "Navy" designations (1851 and 1861 and possibly more) were in .36 caliber. That is, in fact, the actual caliber of the one blackpowder replica revolver (an 1851 Navy style) that I own (so far). The "Army" designations (1860 and others) are in .44 caliber for the front loaders, and larger for the cartridge revolvers (1873 and later).
Now, it is true that the Navy revolvers (as in that
other post) were also manufactured in .44, and the Army and Police models were built in .44 and .36 (also .38) calibers. But I suspect that the primary designation of Army or Navy was based on the originally intended caliber for the model. And I suspect that the calibers chosen were because of weight requirements on land as opposed to onboard ship (meaning land could accomodate a greater weight per shot than ships could). But of course, this is all speculation on my part, and I'd love to hear some better information from folks in the know.
Even if those folks might (God forbid) be sheep-molester Bingley.