TAre you splitting open your calipers for a rebuild?
These small seal rings are used to seal the fluid passages inside the two caliper halves.
It is highly recommended that you replace these seals when the calipers are split apart.
UNIMOG calipers require 2 pieces and the G-Wagen front calipers require 4
pieces on the dual circuit calipers or 2 pieces on the single circuit
Original Mercedes-Benz part.