Keith, I think a lot of those dealer codes are known now.
Here's a couple photos of the Pontiac Retail Store. One is from 1958, and the other is from nearly the same angle, circa 2000. Also a cool matchbook I found at the Pontiac-Oakland Museum.
Mike
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My Bio: I am currently writing articles for POCI's Smoke Signals magazine and enjoy promoting and discussing the history of the Pontiac Motor Division.