Naively I assumed that they were cast solid and then the surface machined to the exact shape, but it sounds like they either have internal passages or they're made of some sort of honeycomb to reduce weight? What else could be inside?
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
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.
The high precision allowed for very quiet screws and that caused all that such a screaming noise back then :)
https://www.nytimes.com/1987/06/22/opinion/submarined-by-jap...
https://www.popularmechanics.com/military/navy-ships/a197847...
