Does anyone know why the norwood buildsheet box codes for a m21 in a chevelle are XJ and a camaro its XG ?? Is this code similar to the WO code stamp on the trans tags that started in 1970?
Yes, it is - that's the Broadcast code assigned to that transmission assembly part number. The only difference between two same-year M21's with different broadcast codes is the diameter of the pressed-on speedo drive gear on the output shaft; there was a "large" or "small" drive gear, depending on which range of axle ratios the transmission was intended to be used with.