I think you will find that the OK to use +p came when the fireing pin was moved from the hammer to a frame mounted one . would expect some other changes took place at the same time but with the hole in the frame leading to the shooters face was a concived problem with lawyers . The fact that other k frame guns had the same design made it seen odd . There are articles that cover the mod 60 having its chambers cut to accept 357 mag back before the gun was made to do so.
I don't blame you for not paying top dollar , if it were me i would get the SP-101, have a trigger job done and spend the rest on ammo.