I am a member of a few gun forums. Today, an individual asked the same question on two of them:
"Is it safe to shoot +P in a heavy barreled Model 10 bought new in 1971".
Argument ensued...one individual even opined that the K frame was never meant for plus P...that's what the L frame was for!
The original poster finally called S&W....Here is his post:
"....Spoke with a rep. in the tech. dept. at S&W. He told me that as of 1982 they heat treated the revolver so that it could take the pressure of the +P round. Prior to 82', the revolver was not manufactured for the +P and he recommended to stick with the .38 special cartridge..."

Oh well...