Hi Tim
Items like the gas filler neck and cap were often just off the shelf Ford and Chevy parts. They didn't want to spend time making tooling for parts they already had the tooling.
This is a long way of saying a locking gas cap for a 1940 Ford might fit.
https://www.macsautoparts.com/ford_m.../gas-caps.html
Now as to how did the military do it and yes they did, I've seen padlock and hasp added.
Though if you put the wire basket strainer back in the neck of the filler tube it is pretty effective at keeping you from putting a siphon hose into the tank. Only problem and why I have removed the baskets from all of my truck is that they make it hard to fill the gas tank with automatic shutoff gas nozzles at the pump.
Of course if gas theft really become in your area, just use one gas tank and fill the other with diesel. Back during the 1970s fuel crunch here in the states one of our local car dealers was being plagued fuel thefts from one dark corner of his lot. His solution was to drain the tank of one vehicle that was parked in that corner and replace the gas with diesel. As the story goes the culprit was found on the side of the road when is car stopped running.
Cheers Phil