MDF. Same as most Atari cabs of that era.
They'll last longer if you don't allow them to get wet. Leg levelers help keep it out of moisture.
The Atari stuff was better than the Gottlieb stuff. My old Reactor (now owned by another member) had epic bottom swell. I think it's because Atari had that grained vinyl with screened art on the outside on both sides of the MDF. That seals out moisture, so you only get swell on the exposed edges unless it's immersed, and then all bets are off.