These are the o-ring seal for the tranny. Verify part number before buying. They are cheap.

016-997-39-48 Governor Cover O-ring
005-997-86-48-M22 B1 O-ring
005-997-70-48-M213 B2 O-ring

I do not understand why you want to change the piston if it works OK. Mission creep sometimes don't work out. It is not difficult ( 6 on a scale of 1-10 ) to change the B2 cover o-ring. W124 chassis is a bit tight but still not too bad. You can always do the B2 when it fails.

__________________
Not MBZ nor A/C trained professional but a die-hard DIY and green engineer. Use the info at your own peril. Picked up 2 Infractions because of disagreements. NOW reversed.

W124 Keyless remote, PM for details. http://www.peachparts.com/shopforum/mercedes-used-parts-sale-wanted/334620-fs-w124-chasis-keyless-remote-%2450-shipped.html

1 X 2006 CDI
1 x 87 300SDL
1 x 87 300D
1 x 87 300TDT wagon
1 x 83 300D
1 x 84 190D ( 5 sp ) - All R134 converted + keyless entry.