I offer for your collective consideration this RPP with Reddish Moldy Spots.
I key 'moldy', as I do not know the composition ...
Q/ Is there a particular reddish mold that favors photographs?
Q/ Is this not mold, but a byproduct of some chemical process?
Found on both the front (photo-reactive emulsion side) and the back, though not thru-and-thru.
Authoritative & ridiculous opinions are equally welcome, but please tag the latter with a smiley so's I know the difference.