Came with several accessories I bought second hand, with a Medela pump. Can someone tell me what this is called and/or what it's used for? The underside of that hard plastic shell is a very thin rubbery membrane. It's maybe two inches across.
hmm...on a lot of Medela pumps, there's a similar membrane, but it's smaller than the size of a dime. In those case, the membrane is very important as it's one of the parts that creates the suction.
You may want to try looking around the Medela site and see if you can find it on their site and perhaps it will have information on what it goes with and how to use it. Good luck!