[COPO]

I don't think the date spread would be too unusual for an uncommon rear end such as a 4.88. I recall the 67 BM Chevelle that I owned had an 11-18-66 build date for the rear and the car had an 06B of 1967 build date, so that would be a 7 month spread (4.56 with metallic drum brakes) and I am 100% certain it was original to the car as documented on the POP and plus other physical evidence.