It could have been done like some jet engine blades, where a thin skin 'pocket' is pressured or vacuumed into shape during forging. Or perhaps simply, they are not made of metal anymore, albeit unlikely.
You're probably closest with the internal passages though; if one of the primary issues is cavitation, there must be some solution to relieve that vacuum pressure from becoming too great, or absorbing the shock when it does
I would suspect that there's some internal structure made of a different material (steel, air, silly putty, whatever) that is tuned to act as a damper or allows the screw to flex in a certain way that changes its acoustic signature allowing it to be quiet at higher speeds like those crappy flex fans for car radiators.
Casting in an internal pocket with sufficient accuracy(!) in a part so massive would have been very, very, very difficult/expensive if not impossible with the kinds of industrial process control that were available in the 1960s.
You're probably closest with the internal passages though; if one of the primary issues is cavitation, there must be some solution to relieve that vacuum pressure from becoming too great, or absorbing the shock when it does