Tanks
What's inside those mechanized fighting vehicles?
Firearms
See the insides of rifles, handguns, automatic weapons, etc.

The Brooklyn-Battery Tunnel was new in 1950, when Alexander Leydenfrost drew this cutaway.  At 9,117 feet, it is the longest continuous underwater tunnel in North America, according to Wikipedia.

Click to Enlarge to 818 x 771 px:

Brooklyn Battery Tunnel 1950

Brooklyn Battery Tunnel 1950

Source:  Popular Mechanics May 1950