O-ring between caliper halves

If you are splitting a caliper in half in order to overhaul it then you will need the small gallery seals that sit
in between the two halves of the caliper.

Early front calipers (for solid brake discs) need one gallery seal per caliper.
Later front calipers (for vented brake discs) needs two gallery seals per caliper.
All rear calipers have two gallery seals per caliper.