Hi Richard,
Jeff, or others, please correct me if I'm wrong.
Richard you are referring to the "Axle Code" and you seem to be saying that is a Plant code for Dearborn. I don't think that is correct. Dearborn, as many other manufacturing plants, were and still produce several similar product lines. Today the Kansas City Ford truck plant produces the F-150 truck and also the Transit van. The truck plants in Kentucky and Ohio produce F250, F350, F450, F550..... Dearborn in the 1960's was producing Cougars along with Mustangs. The cars would have different axle assemblies installed based on a number of factors, including V8 versus 6 cylinder, engine displacement, car built for highway use versus a hot rod. 2:79 gears, 3:00 gears, Lockers, 3:91 gears. All being installed as assemblies on a moving assembly line. Each of those axle assembles was assigned a unique code, such as that 931. The assembly line worker installing axles simply looked at the Build Sheet, saw "931" and took his hoist over to the bin loaded with 931 axles and grabbed one to install. The next car had some other code, so he installed that coded axle assembly. The axle assembly was produced from the axle plant with the metal tag attached with the information on it. Those were meant to be mostly permanently attached. The assembly also had a paper tag wrapped around the axle with the "931" in big letters. (maybe the man on axles on third shift has 20/50 corrected vision....he needs to see that big paper number or he's installing the incorrect axle assembly). Those paper tags vanished as the vehicle was used in snow, rain, mud. Maybe some paper tags were torn off or smudged during shipment to the assembly plant. But the metal tag would hopefully remain to allow a mechanic to quickly see what was supposed to be inside that axle. He could order parts before he even took the assembly apart.
The sub assembly plants, such as for axle assemblies, were also producing multiples of different assemblies. They would have a code for the plant and every axle assembly they produced would have that plant code. But they were perhaps producing 50 different axle assemblies for the Ford passenger car lines (Comets, Mustangs, Galaxies, station wagons) so that 931 axle assembly code was just one of many. That code, perhaps, would only be installed on Cougars and Mustangs.
Marti probably had to correlate what was known about your car based on the VIN. Perhaps they narrowed down the dates and which of a few different axle assemblies were possible based on the information they had.
Your other sources saying 931 equaled "plant code", probably just needed to make a common term that perhaps they use for everything. Chevy, Dodge, Pontiac, Yugo....
I hope I'm sort of correct in this and welcome other comments.
R