Correct me if I'm wrong but I didn't think Colt really built brass frame revolvers. It was my understanding that the brass frame revolvers built on the 1851 Colt design were Confederate "knockoffs" like the Gunnison and Griswold. If that is the case "brass-framed Colt" they used, is close enough in my opinion.
Yes, the idea, as far as the propmasters were concerned, was to approximate the Griswold's appearance. They wanted a Griswold replica, but they couldn't find one at the time and figured that if it had a brass frame and looked Colt-like, it would be close enough. I give them a lot of credit for putting forth the effort, even if the end product wasn't accurate.
Heck, the fact that the producers were not only aware that the G&G revolvers existed, but also made it a part of the storyline was a plus for me.
And, by the way, it really is worthwhile to track down the Fall 2012 issue of Guns of the Old West and read about the work that the crew on the show did to be as accurate as possible.