Formally called the Super X Dance plug - smaller 5/16"
Newer Power Plug larger 5/8" and this is the more common size.

5/16" fits Sure Grip plates Super X, Jogger, and Probe 5/16 style (smaller hole)

5/8" fits Sure Grip plates Probe 5/8, Rock, Avenger, Avanti, Competitor, Century, Classic, Cyclone, XK4, Laser, Power Trac, and Snyder,

Smaller version will fit the Super X plate where the larger 5/8" plug will fit plates that use the standard adjustable toe stops. 

All made in the USA.