Partial VIN on a Pontiac A body frame will be stamped on top of the rear frame rail, usually 12-16" in front of the furthest rear frame bolt. In order to view the confidential VIN with body still on the frame will typically have to careefully remove at least the rear three body bolts on the drivers side, and carefully lift the body a few inches up from the frame. Can then use a small wire brush, a mirror, & a flashlight. It's typically a pain on a complete car. Usually in examining a GTO, or GS for sale, I will look on the outboard side of the lh rear frame rail. In the same area will examine the two letter frame ID code, and the date the frame was produced, that can tell a LOT, esp if one knows the two letter frame code for their particular year GTO, GS. The the date of the frame being produced will typically predate the build of the car by 10 days to 6 weeks. Typically, home Pontiac plant builds are fairly tight dated.
Only in the instance of a build at very low production plant, then the one of the later '70-72 specific code 455 usage frames, then throw in a long UAW strike, will one run across a huge time span between the date a frame was produced and when it actually made it to the line and into a car build. Example... Chris on PY sold a '71 455 coded A body 2 door frame last year that was one of many HW coded 455 frames that was produced in Aug of '70, but due to the low use application, a lengthy strike taking out months of production, and being installed at the low volume Framingham plant, that particular HW frame didn't make it into a Framingham build until Feb of '71. That particular instance is the extreme oddity, won't see it under earlier year A body's, their time gap will be much more normal.