Sheridan valve casing oring (goes on front of valve), new (3x included). For 12 gram sheridans (non cartridge valve). This oring goes on the side of the steel valve casing piece that holds the 12 gram seal.
I have found these don't need to be replaced often. If bad you would notice a leak out of the front of your sheridan (near 12 gram).
New orings. Three are included.