This is the GRS 003-526 "swivel" pin plate with one pin, usually paired with the "fixed" pin plate with two pins, see Otto Frei part number 103.527, GRS 003-527, which is sold separately, see below for link. Pin plates are standard on all GRS Blocks but are options on the GRS Multi-Purpose Vise. Pin diameters are 0.250" (1/4"/6.35mm) for fitting in the GRS blocks and the GRS Multi-Purpose Vise. Holes on top are 0.148" (3.75mm), allowing for use of all GRS Engraving Block accessories. Half oval top is 2-1/4" x 1".
Fits GRS Standard Engraving Blocks GRS Magna Engraving Blocks GRS Positioning Blocks GRS Low Profile Blocks GRS Multi-Purpose Vise