This is a pair of Tube Clamps, they come in three styles.

The Keyed Base Style is a direct replacement for the Tube Clamps that come with the Longboard Racks and Dual Sport 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 and E Bike 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 Double Style is used in the Shortboard Rack Mounts, to secure two bars to a single seat mount.  If you have a Longboard Rack or Dual Sport Rack kit, you can buy these and convert it to a Shortboard Rack Kit.

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.