The part that I understand(and I don't speak Frennch at all) is that it's in your familie for two generations,so it has provenance.
Maybe it is because of the photo flash,that it looks so shiny then.
It,s also because here in Holland there are turning up helmets with these decals that are no good, the decals are ok and the helmets are ok but recently put together.
It looks like someone dugup a pile of RCA decals recently.
Here is one from my collection (left behind in my hometown in 1945) this one has a whole decal and not a combination of paint and decal.