I made a black paper mask some years ago and bought "black manilla paper", it was black right through so where you cut it there wasn't a white edge showing, plus it has a textured surface on one side.
There is certainly no doubt in my mind that the misformed rim on my damaged 8" lamp was original factory finish KG3. This would probably have been November 1939 production.
The mask inside my MT110 was of thin black card. I beleive that there are surviving examples of pre-cut headlamp masks in black card too.
It's difficult to be sure of the card colour in all headlamps, but it is the case that even with black card, reflection on the exterior of a 'Difusa' glass often gives a light colour in a black & white photo.
Thankfully, none of this is relevant to BEF as they painted the glass.:grinning: