I have just ran ethernet cable in crawl space for 2 computers/2rooms and used existing cable box. What is the metal u-shaped piece held with a screw in the back of the box? What is the small metal piece also held w/screw inside box near the hole the cable comes thru? Do I need to put these back on? What are they for? The box does not fall through the hole in wall for use of the u-shaped metal piece.

Thanks!

I suspect what you are seeing is clamps, one at top in back of box and one in the bottom. If you were using electric wires you would want to run the wire through these where the wires comes in and in a multiple branch circuit a wire would leave the box and go to another outlet or switch. The claps secure the wire from be pulled out of the box.

One of the screws might even be for a ground screw.

If you have concerns your cat5 will get pulled out of the box you could secure it with the clamps.

don't crush the cat5 cable with the clamps! It's not a power lead.