The rear end in my 67 only has a 4 digit code rather than a 7 digit. Is this right??

The code reads CON1 or CONI(hard to see cause its all dirty). The CO seems to mean 3.31 positraction, correct me if im wrong.

The ID code and assembly date is a very shallow hand stamping into the front side of the passenger axle tube. It is not located on the center section. The tube has to be scraped clean to see the stamp.

'CO' is a 3.31 posi w/metallic brakes but even if the rest of the 3 data alphanumerics are gone, it should start with 2 numbers. Maybe the 'N' is '11'? What's the date code on the body tag? Try a wire brush on the axle housing to see if there's more there.