I do not know about the exact steel alloy. It is very likely in the range of 4140. 4140 can vary some as to chrome, carbon, moly and manganese. I do know that if you take 4140, forge, NORMALIZE, harden and temper it you get a small increase in yield strength over 4140 that skips the normalization cycle
I don't think nickle would do much for additional tensile or yield strength.
If you had access to a PMI gun you could have a cylinder shot with one and it would tell you the metal composition, minus the carbon as it only detects the metals. I am around them when I work in refineries, but getting caught with gun parts in a refinery now days would get you fired.
I do have a model 10-2 that I made into a 357 by fitting a model 19-3 cylinder to it. The frame have been my experiment frame for years, has adjustable sights a 2 1/2" 357 barrel, round butted. It have several hundred 357 round through it with no ill effects.