two more ideas
It can be argued that any adhesive mounting method is not archival...corners are an alternative that apply no adhesive to the photo (unequivocably reversible, as opposed to the hassle of undoing so-called reversible mounting tissue).
I found an empty box today with a product name to investigate. I don't know if it's archival but it is what I referred to eariler. I didn't empty the box so I can't tell you how it worked.
CODA "Cold-Mount" adhesive coated mount boards
50 - 16"x20" Styrene 0.040"
CODA, INC. 30 Industrial Ave Mahwah, NJ 07430-2207
I suspect it's a PMA board.