Hi Dave,
I very much doubt that that is in fact FRT, FRT would normally only be used on property such as condos and town houses where fire protection is required for the roof decking 4 feet either side of a party wall, also FRT was not used before the late 1970’s.
I believe that what you were looking at was just older moisture damaged plywood which had darkened due to age, damp, and airborne contaminents.
Regards
Gerry