Copper rivet with washer.
Solid copper.
Used together with a rivet setter, for example this.
Available in two sizes; #9 and #12
#9 is 4 mm in diameter and #12 is 3.5 mm in diameter, which also determines the size of the hole in the leather.
Length is about 18 mm below the head.
Please note that the rivets are sold in packs of 10! If you order 1 piece, that means 10 rivets with associated washers.