Every dinghy sailor has seen one of these. It's the ubiquitous Super Spars OEM Mast Step with a 1-1/16 in. (27 mm) x 5-3/4 in. (146 mm) long track.
Insert clevis pins through the holes on the sides of the track to prevent the tenon heel mast plug from shifting.
Use with Tenon Heel type plugs.