Fomac brand dance plugs.  The original USA-made urethane material available in both 5/16" and larger 5/8" sizing. 
Recommended for Jam Skaters.
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 12m