The rear end in my '69RS (01C Norwood build) is a BP code (2.73:1 open) with a 1018 (October 18, 1968) build date.
The rear end in my '66 Chevelle (which I do have the POP) is a CB code (3.36:1 open) with a 1027 build date (Oct 27, 1965). The unit was built at the Buffalo, NY gear and axle plant. The ring gear is also stamped on its OD with the GM part number and the numbers '1137 65'; where 11 is the number of teeth on the pinion gear, 37 is the number of teeth on the ring gear, and 65 is the year of manufacture. The Chevelle was built after 22 March 1966 at the Flint, MI assembly plant and was sold in Decatur, IL on 30 March 1966 to my grandparents.
Although the dates are usually much closer, there are exceptions.