Belgian Post-1854: https://www.rockislandauction.com/detail/69/237
Nice Belgian Licensed: https://www.rockislandauction.com/detail/69/241
Another Nice Licensed Belgian: https://www.rockislandauction.com/detail/69/242
Engraved Belgian by T. Auguste: https://www.rockislandauction.com/detail/69/246
Weird Grip Belgian: https://www.rockislandauction.com/detail/69/243
Belgian w/ Fake Address: https://www.rockislandauction.com/detail/69/239
Another Belgian, Moderate Quality: https://www.rockislandauction.com/detail/69/238
Colt's most important patent was on the linking of the hammer and cylinder, so that cocking the hammer would automatically rotate and index the cylinder (basically, the single action concept). He patented this in the US and throughout Europe around 1836, but his initial business efforts in Paterson, New Jersey were a failure. It wasn't until the late 1840s that he created a really successful revolver design. By this time, more than half of his patent period had passed, leaving him only until the early 1850s to exploit his legal monopoly on his ideas.
European and American gunmakers were making unlicensed (illegal) copies of his guns from the first Paterson days, although they really took off in popularity with copies of the 1851 Navy model. Colt reacted to this by setting up a licensing fee system and hiring an agent to represent the company at the Liege proofhouse in Belgium, where it was possible to intercept most of the illegitimate guns. Those deemed of sufficient quality could pay 10 Francs and be stamped "Colt Brevete", rendering them licensed and the manufacturer safe from legal action by Colt.
I strongly recommend the book "Colt Brevete Revolvers" by Roy Marcot and Ron Paxton: http://amzn.to/2e9t73O