After removing only the mirror glass panel from the metal cable-connection panel on an original drivers side mirror, I discovered the date code etching is within the "silvering" layer of the mirror.
I installed JUST the dated glass into a reproduction mirror assembly and side-by side to the original FoMoCo one, besides the pitting on the original one and no numbers on the base of the aftermarket one...I think it passes muster very nicely
Thanks everyone for combined wisdom
The 3rd picture is of the original glass (just the date-coded glass) after being installed into the new reproduction mirror assembly and the next 2 pictures are of a side-by side of the original with this re-pop mirror. (FWIW, The new mirror I used was an 'old stock' Scott Drake mirror I bought many years ago from somebody selling leftover parts on ebay.)