The Signature Series(aka 3rd Gens) were made by Lou Imperato's company, Iver Johnson. Iver Johnson had already had the rights to make the F-Series 2nd Gens when Colt abruptly discontinued the entire line. This left Imperato stuck with a whole bunch of unfinished guns and probably his suppliers as well. It's likely Imperato threatened legal action but whether he did or not, eventually he and Colt came to an agreement whereby Imperato could make and sell what became known as the Signature Series Colt's.
The main difference between the Sig Series and the 2nd Gens is the 2nd Gens were inspected by Colt, the Sig Series were not. Also the Sig Series included models and variations the the 2nd Gen did not, including the Paterson, four distinct versions of the Whitneyville-Hartford Dragoon and the Military Version of the 3rd Model Dragoon, with the recoil shield cut for shoulder stock. Numerous special editions as well, such as the Tiffany 1860 Army.
Regarding the Belgian Brevette's. Samuel Colt met with a consortium of Belgian gunmakers as a way to fend off potentially difficult, expensive enforcement of his patent. At one point Colt had a man in Belgium whom any gun manufacturer(not just those of the consortium) could bring guns to and if they passed a quality control inspection and paid a fee, they would be labeled "brevette" and could be legally sold.
