Latest version of the XT Omega Nub.  These are now factory made from silicone rubber, has the same dimensions as the official Maple Leaf nub but is much cheaper.  Primary used in conjunction with Maple Leaf buckings. 

Comes in 2 different types of hardness; 70 shore (Red) and 80 shore (Black).  The original is very hard, roughly 90 shore, this can make fine tuning difficult with some hop systems.

Red is recommended for 50, 60 bucking hardness
Black is recommended for 70, 75 and 80 bucking hardness

This is only a rough recommendation since every hop chamber applied a different level of pressure.

BB weight and hop chamber also plays a part in deciding which hardness works best.  Sometimes only way to tell is to experiment.