One dark side of the collector car hobby involves illegitimate vehicles. Cars that start off as a lower/base model and then restored as something much more desirable. Fairly common with Chevrolet cars since there isn't a reliable authority to validate their cars. So a buyer of a big block Chevelle SS doesn't really know if the car is authentic. Original dealer invoices, warranty docs, registrations, titles etc go a long way to authenticate the car.
Fortunately, the Pontiac community isn't in that same boat. A fellow enthusiasts who worked for GM was granted permission to save much of the factory invoices when Pontiac was planning to toss those records. I am not sure exactly which years he has records for but I think the range is early 60s through early 80s. The guy with the records is Jim Mattison and the go to person to authenticate Pontiac cars from this era. His website is PHS-Online.com (I think..). If that doesn't work than just Google it and it should come up. All you need to do is fill out the form and submit your VIN and he will send you a copy of the original dealer invoice which shows how your car was built (model, body style, color, trim and other options).
I think the fee is $60 or $70. You will also get other useful documents besides just the invoice. He will send model year specific dealer order forms and reference docs that you can learn more about the car.
Not all GTOs can be identified by just looking at the VIN. Only the middle years had a special model designation "242". 1971 is the last year for that so your car is a GTO. If you had a 1972-74 or an older 64-65 model then the only real way to know if the car is a GTO is to check the PHS docs. This is because the GTO was an option package on the base model during those years.
Hope this helps.
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