JBL MTC-426MR - Plaster-Ring Bracket for Control 424LP / 426C/T / 426LP / 426C/ T-LS (6 Pack)The optional MR PLASTER RING BRACKET (or Mud Ring) contains a circular offset, forming an edge guide for sheetrock plastering. The bracket has wings that attach to the building structure. Sheetrock is typically either precut or cut with a rotary cutting tool using the outside of the plaster ring circle as a guide. The sheetrock hole is then plastered (or mudded) up to the ring to create a seamless cutout.