69 Camaro 12-Bolt Posi

[69L78]

I have a 12-bolt posi that came out of a 69 Camaro, but all the numbers don't correspond for a 69. The code on the axle tube is BS W0221 E, which shows it's a 3.31 posi for a 69 Camaro. The center section casting # is 3969341NF, with a F180 date code, which says it's from a 70 Camaro. The dimensions of this rear is definitely for a 69; the measurement between the axle tube flanges is 54 1/4", and the measurement between the centerline of the spring seats is 42 7/16". Can anyone explain this? By the way, if anyone is interested this rear is for sale or trade.