Am having trouble IDing an Olds rear end/differential. It's 5 on 5 bolt pattern with left hand threads on driver side. The unit is in a 29 Ford RPU that I recently purchased. It has railing arm suspension with coil springs. Width has been shortened to fit the Model A chassis. Looking at the front of the differential I see a "13" at the 11 o'clock position, "F201" at the 5 o'clock position and __1326__" 1st digit looks to be most likely a "6". Least likely a "5". Last digit appears to be a "1". Any help greatly appreciated as I need to know what I'm working with. Thanks