These black seals bridge the C02 Canister to the beer machine and ensures that there is an air tight fitting not to allow gas to escape. These are high quality seals and are compatible with most 5L Machines. The material used is a specially designed to withstand very cold temperatures such as those produced when in contact with C02.
Never change the seal when there is C02 still in the canister, as you could come in contact with the C02!