You'll know if you can play it first whether or not it was stored in a damp environment for long periods of time. I've tried some 70s BB cabs that sounded like ass and smelled damp...those were shot. Since I've had a cab just like that, same vented 75s with that same white on the frames, I don't believe that is evidence of damp storage since that cab I owned sounded as killer as the other vented 75 cabs I've had.
For 650, that's easily a good deal IMO since they only put the vented 75s in for a year or so; 84-85 then non vents. They sound WAY different than any other 75, almost like a 65.
Those cabs are worth 800 or more depending on the condition of the cab.
I should mention the screws...any vintage cab I've bought, I've had to re tighten down many screws throughout the cab. Over time and depending on how they were used, those screws loosen. It's good to check all your cabs as they all might have a few screws loose lol