I have a unique book called "Ford Muscle Parts Identifier and Restoration Guide". It was printed in 1992 by robert Warner after years of research. It is another reference point for our hobby and contains a plethora of information on high performance Fords from 1965 to 1973. Especially tags.
The VIN number is pretty much indisputable for your tranny. And the tag number PGA P2 is referenced in this book as a 1968 GT-390 tranny. BAM! Done deal. As tags are easily lifted and moved, the tag is just a number on a tag. The real deal is the VIN and the cast numbers. When these "all" align authenticity is golden!!
Rob