I took a look at the MC bits and pieces and here's the answer:
That sheet metal cup shaped piece fits with the bowl-side to the rubber.
There's a piece missing from your photo that then fits against the cup. This piece is like a stepped solid steel washer; the center hole fits the end of the piston assembly, and the raised part centers and locates the spring.
Wanted: Dead, smashed, crashed or trashed gauges
BMW GAUGE REPAIRS - TACH*SPEEDO*CLOCK*VOLT METER *PODs & LIGHT BOARD*