Have you tried searching the NCRS Dealer Code Database yet...here's a link if you haven't.. If you need to be a member to use it, I'd expect someone here is and would be happy to help?.
http://www.ncrs.org/dealercode/dealerzone.html
Also there's a GTO site w/ a bunch of Dealer Codes listed...I don't see yours there but considering a lot of Dealers were Chevy/Poncho franchises back in the day, here it is anyway if others want to search or bookmark it..
http://ultimategto.com/art18.htm
Good luck!.
~ Pete