I obtained a LOT of information re 'Dealer names/codes' and zone numbers from questioning a long time friend of mine who has worked in 4 or 5 of the north alabama dealerships in the PARTS Department over 40+ yrs. It seems the *parts guys* know more about these changes than anyone else in the dealerships as all the parts that come in to them have the zone/dealer number included. I 'suspect' that GM uses/used the zone/dealer number for shipping/billing of everything shipped to that dealer (cars and parts as well). For the Huntsville Alabama dealership (that has changed owners/dealer names a half dozen times since then, the *dealer number changed each time. For all the north Alabama dealerships, the ZONE number also changed several times due to the *warehouse location* changing over the years (It's been in Birmingham, Atlanta, Mississippi, and now is in Texas I think).
So the point of my 'story' here is to find an OLDer long time parts department manager or employee, they *might* be able to recall the history of the zone/dealer numbers for your dealership of interest.