Definitely could be a fake stamp, psa have graded fake stamps before. I’ll have to look at some pictures and really compare before I jump to conclusions, but I would be very skeptical of that Blastoise. If it is something with the trap, Tavis would have a lot more knowledge about those stamp variations than I. Sadly, I don’t own any first edition foils to truly compare for myself
The only other difference that I’ve noticed is the color of the “HP”. Thick stamps with the -15 backs have a different shade of red than the +15 backed foils. Its subtle but all ive been able to find