Hi There,
I have typically seen PIO from the Metuchen plant for GT in 1965 as well. However, the Marti Tag book indicates that PI and PIO were both used. Do you have a photo of the tag you could post? If it's a reproduction the inspector "punch-outs " on the tag would be missing. I've attached an example of a buck tag with the inspector punch-outs. In addition, I did find an example on-line of a 1965 Metuchen tag marked PI (I've attached it to this post as well). One of the challenges with validating this particular car is the door tag is missing. However, provided that the VIN stamping in the front fender aprons matches the buck tag and the buck tag appears to be original then that would at least open the possibility the car is a GT.
Does the car have the other typical factory GT indicators? In this case, since it's an "A" code, it would have come from the factory with single exhaust unless it was a factory GT. As a result, if this car is a GT it should contain the reinforcements to the rear floor pans under the rear seats to accommodate the dual exhaust hangers. If those floor pan reinforcements for the dual exhaust are not present, then it is not possible to be a factory GT.
Regards,
Ron