This washer is used on trombone slides as a top out bumper. It's made of green felt, .080" thick. If the top out bumpers inside the cork barrels on your trombone have compressed, you can use these to build it back up again. You can tell it's time when your slide lock mechanism is banging into the cork barrel and is making a distracting racket. Your section mates will let you know.
To build up your top out bumper to its previous thickness, use one washer on each slide tube. Simply fit each one over the end of the inside slide tube, then slide it up to the top. To insert into the cork barrel area, place the outer hand slide back on and use its tube ends to slide the felt to its home.
If you're replacing the slide springs on an vintage Conn trombone with felt, you may need several of these washers for each slide tube.
If you need all of your old top out bumper padding removed, we have a tool in our La Crescenta store and can do it for you. Do stop by.