This screw threads into the center rotating rotor post, and is used to hold the rotor stop arm (armature or bell crank) in place. Used on various John Packer rotary valve instruments, including horns, trombones, bass trombones & tubas.
The Stop Arm (also known as the Armature) is the piece that connects to the rotor shaft and moves the rotor back and forth. This screw holds it in place.