Just as an aside comment, we do find holes behind the wappen that were plugged/filled in during the war. However, it was never done like this with "finish" slopped on the inside of the helmet. A period repair should have a leather plug in the hole, finish applied to it on the outside to fill in and finally a circular thin leather patch glued over the hole on the inside. One other faster method I have seen, is a black painted, round flat head split prong metal brad. This is just inserted into the hole and the prongs bent back on the inside.