Yes, it's 12 and 13 in the diagram. They're the inner and outer piston seals. I don't have the transmission manual available here at this time (it's on a computer that is down at the moment) but I remember something about noise in reverse possibly being caused by the B3 piston having to move further than normal and contacting rotating parts of the transmission. This is normally due to excessive B3 clutch wear, but could also be an assembly error. It happens to the best of us when working on transmissions

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Erich Loepke
2010 Ford Focus
Currently Benz-less