I don't have my SCSW handy but I believe -3 and later Model 12's had the same frame width as their all steel M&P counterparts. I'm sure someone will correct me if I'm wrong...
OK, you're wrong. It is the 12-4 that is the same width as the steel K-Frame guns. There is nothing after the 12-4.
Another common error, and
you did not appear to make it, is that the grip frame of the Model 12 series is thinner, this is not correct!
All K-Frame revolvers have the grip frame the same thickness. It is the major thickness/width of the frame that is .080 thinner than the steel frame guns.
Your information on Aluminum cylinders is incorrect too. No model marked Model 12 ever had the Aluminum cylinder, this is a warning and complication that does not need to be given. Aluminum cylinders were only used from 1952-1954 in the .38 Military & Police Airweight. This
is not a Model 12. It was three years after the Aluminum cylinder was discontinued before this became the Model 12, in 1957. Your remark with the photo of a blue M-12 is non-sensical. While true for a blue gun, there is no difference in the Nickel between the frame/sideplate and all other parts of the gun which are steel. The Nickel doesn't care what base metal it is being applied over.
lay2004, your Model 12-2 is completely safe to fire with all standard pressure .38 Special ammunition, as long as it is mechanically sound. You should take it to a local gunsmith to have it inspected to verify this since we cannot examine the gun "hands-on".