It's impossible to know for certain when it was done of course but there are plenty of images of the semi-circle cutout paper masks fitted upside down (or half rotated). I wouldn't rule out either that someone had fitted the reflector upside down before painting it.
Painting half the reflector black was a government decree early in WW2, I have a bicycle light painted like that, as the war went on the rules got even stricter, so I suspect this is probably original.
