8131 was not there. Almost impossible for it to have been anything other than a 4-5/8" gun at that time, unless retrofitted, of which there are a handful. The 6-1/2" guns don't begin to show up until like serial number 22000, two years later.I'm not sure any of the ones I owned/own were entered.
8131 - Don't remember length I think it was a 6"
8567 - 4-5/8