I never stop learning. Had a need to find some plain head under seat hinge and locking tang bolts to fit to a new Yamiya repop seat. As some will know, Yamiya supplies the Tang and new bolts with their seats, however originals did not have any #8 on the head which Yamiya ones have. Anyway, I have 2 original red foam seats which have the original hinge hardware. On removal of one set of bolts I noticed thin black washers fitted to these bolts. I decided to check the second seat and sure enough these same thin black washers were fitted. They are made from a plastic material and I can only assume they were fitted to save the black paint from being scratched by the split washer. Here's a photo