Thomas & Betts 531511234-20R 4 SQ 1 1/2 BX EXT RING 1/2 3/4 KO

The 531511234-20R 4-inch square extension ring adds 21 cubic inches of usable volume to the box it is mounted on. That is the number you add to the existing box volume when you run the NEC box fill calculation, so it is the fastest way to get an overcrowded 4-square box legal without opening the wall and replacing the box itself.

Yes. This is a standard 4-inch square (4-11/16 is a different size) extension ring, 1-1/2 inches deep, so it bolts to the two mounting ears of any standard 4-square box using the same 8-32 screw holes. It raises the box face forward 1-1/2 inches, which is what you want when drywall, tile, or a second layer of finish has left the original box set back from the wall surface.

Different jobs. This 1-1/2 inch extension ring is an open, full-depth collar that adds 21 cubic inches of wire volume and brings the whole box face forward, and it has 1/2 and 3/4 inch knockouts so you can land additional conduit runs at the extension. A mud ring adds almost no volume and only reduces the opening down to a device size. Use the extension ring when you are short on box fill or need to make up the depth of added wall finish; use a mud ring when depth and fill are fine and you just need a device opening.

There are 12 knockouts, in combination 1/2 and 3/4 inch sizes. The combo style means you punch the smaller ring for 1/2 inch conduit or EMT connectors, or take out the full knockout for 3/4 inch, so one ring covers both trade sizes without stocking two part numbers.

It is steel with a silver (zinc-finished) surface, so it is a conductive metallic ring. Bonded through the mounting screws to the box behind it, it stays part of the equipment grounding path, which is how a metallic raceway system is normally bonded. A nonmetallic ring would not do that.

It is sold as a single ring, one each, and you should plan on ordering the flat blank cover or device cover separately, plus a plaster ring if you are mounting a device at the finished face. The 8-32 screws that attach the ring to the box behind it are the same ones already in most 4-square boxes, but for a clean install order hardware and cover with the ring so the job does not stop.