O-ring 02031 EPDM
Almost too many applications to list here, but used primarily with Brasilia water inlet and Rancilio and San Marco retention valves. This is the same as part M_40 or M_41 listed elsewhere.
I replaced the pump on my expresso machine .. and noticed the ORing between the pump and water outlet line was leaking .. I ordered new ORings .. I had to tighten the connection far more than I had expected .. to the point that I flattened the ORing to keep it from leaking .. which is what the original ORing looked up when I removed it .. everything is working perfectly now ..