1958 2nd gen. Bought this one and had them stamp it 44 Colt. The USFA cyl chambers 44 Spl.
From my Colt SAA screw threads by generation vs. USFA and Ruger:
1.) BARRELS: 1st gen, 2nd gen (and USFA/all 1st Gen clones barrel threads) are the same at 20 TPI x .695” + or- tapered thread, while 3rd gens are 24 TPI x 11/16” (.681” + or –) (the same as Ruger SA mid-frame and large frame barrels with the exception of the SBH Hunter models, and a few others which vary).