I believe the 1877 Colt was the first double action model. It came in .38 and .41 calibres, and is fairly famous for its too-delicate design. There's a complicated little spring that tends to break with time and use (but there are people in the states who will be happy to sell you a replacement). They're neat old guns. Billy the Kid was supposed to have had a .41 "Thunderer" in his hand when shot by Pat Garrett.