Look on the right side of the barrel as it could be a .38 S&W (not .38 Special) to be sure. I have 31xxx and it's from 1928 in .38 S&W
Just a pointer: You can tell immediately that this is a six-shot cylinder and therefore must be a .32 RP by the position of the cylinder flutes.
Compare the OP's gun (left) with my .38 RP (right) in the attached photos. On a six-shot, the middle flute will be centered.