Okay. I'm going to retract my comment about the barrel.
Take a look at the five screw revolver in this picture. It shipped in 1951, with a 6" heavy barrel. You can clearly see that the location of the cartridge designation is in the same place as the one in the OP's photo.
As previously established, the OP's frame is a K-38 Heavy Barrel Masterpiece frame that should be wearing a 6" barrel.
Ergo, it almost certainly has a cut down six inch barrel.
