On the face piece attachment. By looking at the photos available,
1-On the flare of the helmet (outside) there appear to be 2 rivet like pieces at the point where the face plate would be attached.
2- The "rivets" may be modified bolts held on the inside by thin nyloc nuts.(this would allow for tightening the bolts and leaving some play).
This is what I see. I maybe totally wrong but I feel strongly that the image shows a nut as I see two of the flats in the image.
I like your idea of the nyloc nut leaving play as it looks like they had issues keeping it from falling down. It's definitely really thin, it might even be cast into the resin piece. I've looked at all kinds of brass fasteners and can't pin it which makes me think its a specialised item for leatherwork or upholstery etc.