Fomac brand dance plugs.  The original USA made urethane material available in  both 5/16" and larger 5/8" sizing.  Sold in pairs.

 5/8" - larger threaded plugs for use on the Probe, Rock, Competitor, Classic, Cyclone, XK4, Snyder, Laser, Power Trac, Avenger, and Avanti plates.

5/16" - smaller threaded plugs for use on the Super X plates (less common)

  -actual production colors may vary from the pictured dance plugs

Stem size 5/8" or 5/16"

Footprint 22mm

Thickness 12mm