EURO TOOL riveting block. Dimples on the left (C) are marked with wire gauge sizes 18g, 16g, 14g and 12g, and are used for support when riveting, to hold the shape of the rounded rivet head. Circles on the right (D) are for setting eyelets (tube rivets) in three different sizes without flattening the rolled flanges. The smallest circle fits 1/16" diameter eyelets; middle circle fits 3/32" eyelets. The middle row of holes (B), also marked with wire gauge sizes, are approximately 1.8mm deep, helping you cut the right length of wire for a nice rivet head on each end. 
There are also dimples and circles on the edges of the block that allow you to rivet and set eyelets on curved surfaces (see alternate image). Designed by Eugenia Chan of Eugenia C Design.