Just figured I'd follow up with this thread to help others in the future, as I've had one heck of a time gathering this information from bits and pieces of different forums, articles and my own car.
I ran across paint code PPG 33053 "Medium Gray Poly", had it mixed locally in a SS Urethane, compared it to the back sides of my rocker moldings, and am very pleased with the match. I believe the existing paint on the backs of the moldings is original, as it's a low mileage all original car. The color is also consistent with the paint under the removable letters and chrome trim pieces throughout- which have never been removed before me. Attached are pictures for comparison showing the backside of a few different pieces of the rocker moldings. Also attached is a comparison of the PPG and the AP-GPA sold by NPD.
The only caveat to the PPG 33053 is the finish- most SS urethanes are a glossy finish, so I will have to bring that down to an "eggshell" or a "near sheen" by having them add flattener. I will update this thread on the amount of flattener used on the final batch when I get a few more samples mixed up and sprayed.
More to follow,
Matthew