There is a date code on the barrel flats and the bore in mm just forward of the flats; and possibly stars indicating the choke
.410 = 10.4 mm; 28 gauge = 13.84 mm

1965 catalog repro
https://cornellpubs.com/product/379...sTqM6UfPMSz39-LEtXUOqO6eDjYWKLPOtNbRcOJk