Hawg and Denster are correct. The C Series 2nd Gen Colt's were made under the Colt dome from rough castings from Uberti. The later F Series 2nd Gens were made at the old Iver Johnson factory in NJ using Uberti parts and shipped to Colt for final inspection. Colt will letter both.
Lou Imperato was involved with Colt on the F Series guns and when Colt decided to quit making them in, I think 1982, Imperato was stuck with a lot of parts and tooling for the guns, so he talked Colt into letting him make the Signature Series revolvers by licensing from Colt through his Colt's Blackpowder Arms Co starting in 1994. According to the Colt historian, Colt initially lettered some of the Signature Series guns, but as they did not have a list of the serial numbers and date of manufacture they quit lettering them.
Before his death, Imperato stated that the Signature Series guns used rough cast barrels and cylinders from Uberti but made the frames and small parts including the screws at his Colt Blackpowder factory in the U.S.A. I have both C and F Series 2nd Gens and Signature Series revolvers and the Signature Series guns seem to be a little bit better made, although some exhibt over polishing. All of the 2nd Gens and Signature Series guns have the short arbor problem.
Fingers McGee, a poster here, is the real expert on these guns and may be along soon to add more.