3/8" diameter universal (dome) head aluminum drive rivet, grip range .125"-.250".
The smaller head variant of the 1" diamter head rivet, which has the advantage of blocking less space on the sign when installed.
To buy the washers used with these parts, enter "NW12-6" into the search bar at the top left of the site or go to the "Sign Installation Rivets & Washers" category.
Part Number: DRAU126AB.