Well, from what I remember, and it has been ages since I worked on one, they may have sealed the whole vehicle up to and including the radiator air outlet, so that may be some form of air outlet bladder with a relief on it to allow air to pass through the radiator.
I do remember the driving position, and would hate to be in that one. I take it the whole thing was sealed maybe up to turret ring level, and cooling air was drawn it through the hatch and blown out via that bladder maybe ?
Cold, wet, nasty, and about to get a very severe dose of salt water rust, poor thing.