This is a pair of Tube Clamps, they come in two styles.
The Keyed Base Style is a direct replacement for the Tube Clamps that come with the Longboard Racks. They are keyed to prevent rotation on the headtube and seat post mounts.
The Flat Base Style is a direct replacement for the Tube Clamps that come with the Moped Racks. They have a flat base since there is no keying on the Moped Racks. These are also great for deck mounting to any flat surface.
The clamp size is 1.25" Diameter, which means they can fit 31.8mm handlebars and Carver/Electra Surf Rack Bars, and anything else that is 1.25" Diameter.