PHS is Pontiac Historical Society. I believe if you go to phs-online.com you can check it out. I'm not sure on the web site, but you can do a search and find it. If there is a GTO you're wanting to buy, you can fax a form with the VIN to PHS and they will send you all the info on the car like the bill of sale, etc. It'll show you the motor numbers, trans numbers and the options the car came with. You can opt to pay the extra if you're in a hurry for a fax back info in 24 hours and they will also send you a packet. When I found my car, I put a down payment on it by phone and flew down the next day. While I was there, the info was faxed to me at home and I could check the numbers while down there. I believe it cost a total of $65 dollars to do that. Even though that price went up, it's still nothing to pay to help make the right decision.
Hope this helps. Good luck in the hunting.