Hello Simon,
you wrote that you've found red crosses under the paint:
The bonnet has a big red cross under the paint, and the rear panel, and rear 1/4 panels also have red crosses painted over.
You've got an ambulance roll over bar, two holes at the side, ... -> Your Iltis was certainly first produced as a a medic vehicle (You've also got no trailer hitch - do you?).
And later it was used in other content -> Therfore they put rear seats in, ...
I think this is (one part of) "the story" of your car.
FYI: On this picture you can see an "usual" roll over bar of an Iltis (= "round" back brackets):
Example