I rarely ever have contradictory info to what John posts, but this time I do.
At least in part. The Database of Dealer Codes I've assembled, consists primarily of dealers who were in business fron 1967 to '76. So perhaps that is the reason for my different observations. The "corruption" of the original plan may have been well advanced. But it appears we are making too much of the possibility that dealerships were numbered in any kind of logical order. Of the 339 dealers on my list, from all Zones around the country, there is no order to be seen, alphabetically or otherwise. As has been stated, dealerships came & went with sometimes surprising frequency, or again as has been mentioned, names were changed without the code neccessarily being changed. So to look specifically for an "F" or "G" dealer name may be limiting the scope of the search too much.
The closest to 310-185 in my list is 310-197, Greenway Pontiac-Olds. INC. in Morris IL. (12-71), so at least at that time, the possibility of an "F" or "G" still looks plausible. I record every date with the dealerships info to try & see if eventually I can build a rough aproximation of how long a particular dealer was in business. Wishful thinking I'm sure, since I primarily focus on '67-76 Firebird & '69-72 GTO.