A friend of mine has possessed this cap & ball revolver since the mid 1960's. He obtained it when he was in the Air Force stationed at Bitburg AFB (Germany). Apparently he obtained it from the gunsmith at the shooting range there.

I think (key word) it's a copy of a Colt 1851. It has a barrel length of about 3.75 inches. The trigger guard is not brass. It has some naval scenes engraved on the cylinder. Several 'proof marks' are visible, scattered on the barrel, cylinder, and a few on the frame. The most prominent is a crown over what appears to be two keys (?). It's also marked 'Cal 35' on the frame. On the barrel is is marked 'Cap System Made by F. Dumoulin'

The barrel, frame, triggerguard, and bottom of grip frame are all matching serial number. However, I found on an auction site from a few years ago a similar pistol being sold with the same serial number.

Any idea what it is?

https://imgur.com/a/M6MsasU

It's a Belgian copy of a Coplt 51 Navy. Some Belgian 1860s sold by Centennial Arms. (Centaure marked). In the 1960s were imported; I have one of those. Belgium made Colt revolvers during our Civil War.
All 51 and 60 revolvers have the Naval battle scene from the Mexican war.
Yours looks to be an original from the 1860s and is worth $1000. More or less. Made by Dumoulin. Sold to the Confederate Govt. These are rare, but you have to find someone who collects them.

The term "cap n ball" is a generalization back in the day and today, of a firearm capable of shooting a lead "ball" ignited by a muzzle loading exposed primer in a rifle or pistol, at least in my art of the USA.