The A-side is the opposite side of where you'll find 3B.

1. Pry open the rubber cover.
2. Unscrew the nut with a 8 mm wrench.
3. Remove the metal lid.
4. Bend off the metal collar.
5. Pry open the rubber membrane.
6. Remove the metal lid.
7. Remove the spring.
8. Take off the locking.
9. Remove the plastic disc

You recognize the A-side by the square around the small hole (A).

Opening a height corrector

It's a good idea to put all the small parts in sequence in such a box, then they are not in disarray if you bump into them.
Clean thoroughly with paper towels. Blow through the small holes on either side with compressed air.

1. Pry open the rubber cover.
2. Unscrew the nut with a 8 mm wrench while holding 3B with a 10 mm wrench. If the nut is hard to open, set the 10 mm wrench in a vise.
3. Remove 3B.
4. Remove the washer.
5. Remove the metal lid.
6. Bend off the metal collar.
7. Pry open the rubber membrane.
8. Remove the metal lid.
9. Remove the spring.
10. Pull out 10B.