5 x 5/8-11 Angle Grinder Back Up Pad.  This pad is made of solid high quality rubber, just flexible enough to polish & sand properly.  The top of the pad is 5/8-11 thread that fits most any grinder.  Each pad does include the disc nut as shown in the photo.

This back up pad is for use with 5 x 7/8 resin fiber sanding discs.  These discs usually made of aluminum oxide but can also be zirconia or other materials used for grinding, sanding or polishing metal, stainless or aluminum.