Model 31 would be a Regulation Police in .32 S&W. From the serial number, I would guess you have a J frame, possibly a model 37 and you have mis-read the 7 as a 1. In that case it was shipped in 1969-1970I have a Model 31-1 38 special Serial number J12208 I cannot find any information on this gun at all chambered in 38. Does anybody have any information they could share with me