After seeing the W-27 complete axle for sale in Springfield at the Nat's with the SX code I decided to crawl under my W-31 and look for a stamp. Using the W-27 axle (pic below) as a reference I located a 'TO' stamp upside down which based on my documentation and stampings on the rear is correct for the car.

 

The stamp TO is correct for a '70 anti-lock (posi) 3.91:1 gear ratio rear end specific to '70 W30 & W31 except with W27.

 

Also correct for '71 and '72 anti-lock 3.73:1 gear W30 only