This 'cold galvanizing', as it's sometimes referred to, is 97% zinc powder in in organic binder.  One common brand is Galvacon, trademark of Lanco Paints.

If you look at something that's galvanized and has a weld in it, and gray-colored paint over the weld, that's it.

Note that although 'you can't weld something that's galvanized', of course you can.  You just have to grind off the zinc coating for a half-inch to an inch away from the weld lane, then paint over it with this stuff.

Note that I have no experience with something large like a car roof being painted with it, but perhaps the shop couldn't easily get that much steel galvanized during wartime, but could buy the paint.

Ron Merrick
not a paint specialist