The Mitee-Bite fixture clamp is a screw with a head diameter eccentric to the shaft of the screw and a hexagonal brass collar that fits around that head. Wholesale Tool has the 1/4″-20 thread Mitee-Bite Fixture Clamp for $5.35. Sound boring? It might be, until you need one.
When placed against the edge of a workpiece (that has a stop opposite the clamped edge) the screw is turned and this clamps the part. It’s a simple, low-profile way of clamping parts for machining.
Mitee-Bite Fixture Clamp [wttool.com]