Note the BBC modifications on this image of this board - the extra wiring and relay to provide PFL on aux 3/4 muted by local mute, and aux 3 and 4 are disconnected at the insert points V1 and V3. Also note that this is a "B" version - RV4 and S4 are not fitted (bottom left of the image).

The relays that provide volt free closed contacts for local and distant mutes - 2 sets for each of the mute circuits. Used for controlling "Mic Live" indicators, etc.