Ok I know it's only a washer and no one else will ever review it BUT IT'S A GREAT WASHER :-)
Most of the other washers are too soft and deform. Particular problem on an otherwise superb Whaleshark pump. These are nice and firm, and they have a U shaped cross section where the groove fits into a raised section on the connector. So it keeps its shape, seals perfectly, and doesn't move. I'd tried several types I had lying around and this was the only one which didn't let some air out. Thank you!