The opening is just part of the regular body pressing, I'll add a pic if I find a clear one. It's just that certain models, or TA specifically, had a plate screwed over this opening. I don't know if it was also sealed or what. No diagrams or references yet, hard to google the plates or a stripped car showing that area. No manuals show it yet, still have a few more to dig through Ike the 67-75 parts and smaller editions of.
Does the answer even matter, not terribly or at all, but now and then things pop up that you've never heard of or noticed that others are well aware of and never mentioned. Maybe I'll google Mexican contraband hiding places...
I just picked up a set, not arrived yet, as part of a collection of used pieces. I had to ask the guy, who in turn asked his wife, what they were! I just thought random GM whatevers thrown in, not hard to cut or make if ever needed to. Not that you'd ever need them....until you realise you are missing them, then they are the only thing you need, just because!