This is a replacement #120 electroplated diamond disc (all steel, no magnetic rubber) for the double-sided discs sold after January 1, 2022 and some discs sold in 2018.  When the original disc wears down pry off the coarser side of the original disc, clean and gently sand the back side of the remaining fine diamond disc with the magnetic rubber until it is smooth, clean and flat, then attach the new disc on to the old fine disc with the magnetic rubber, using the bat pins as a guide.