Sealing O-ring for the outside of the Katadyn quartz sleeve holder, nominal 66.27 x 3.53 mm. Elastomer seals in an ultraviolet reactor work against warmth, pressure and direct UV exposure, which hardens and shrinks them over time. They are replaced whenever the joint is opened rather than reused: a seal that looks sound can still have taken a compression set that will not re-seat. Katadyn part reference 1193-1208.