2022 update : my Pontiac dealer list is currently about 4800 coded, and about 9900 total. I continue to work on this list regularly.
Some observations ~
• Not positive PMD skipped numbers to maintain future alphabetizing. I have some runs of numbers which suggest they didn't (470, 471, 472, 473, 474, 475, 476, 478, 479, 480, 481)... but mathematically the system allows for a far greater number of dealers than Pontiac ever would have (27 zones x 999 codes = 26,973 dealers), so... there was plenty of room to. But via observation, I haven't seen a clear pattern yet.
• The zone/dealer code system got some modifications right about 1987.
Some zones received new dealer #s starting 'high' numerically. Zone 10 is one; around 10 600 starts a new batch of dealers, not all of them correctly alphabetized.
• Some dealers with the same name/address appear in 2 different zones; the 'divider' there being... the year circa '87. Wright Motors of Evansville IN is in Zone 12 (sources: '69, '75), but is also in Zone 9 with an '89 source. This may be a circumstance of the '87 mods, when it seems a few zones 'went dormant'.
An example of this is Zone 12 - I have no newer source years for the dealers in 12 than the mid 80s (unless it was in business unchanged since before that, and continued). In other words: there appear to be no newly-issued codes in Zone 12 after '87. There's a few more Zones like 12, post 1987.
I think we'd certainly like to have a rigid, well-defined system here, but GM has a long habit of... not doing that. If you get a chance to see a list of GM's XP-project vehicle numbers, they did it again there.
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