Hey, those weren't asbestos shingles were they? I'm thinking perhaps concrete but not sure. BTW, you are correct in that the exterior wall has/had a lot of water damage. Termite damage will be associated with mud tunnels whether broken or intact.
I don't believe I've done one that had the old diagonal boards as an exterior wall. Done plenty where diagonals were used as flooring. those are a pain!
...JP