Yes, it's the MFS selector.
A frankly bizarre system which used a fixed, but manually rotatable compass card. Most pilots prefer the aircraft heading to be at the top!
I can't remember what 'Datum' did - but most of the other selections are reasonably obvious and affect such things as the Azimuth Director Pointer demand rate. I don't recall ever having used the 'Mach' selection; presumably it was used for cruise climbs at constant IMN when such things were permitted?
Autopilot and Heading Reference System / Military Flight System selectors were on other panels.
The HRS used an ex-Lightning Master Reference gyro. A pity that the MFS displays weren't thrown out and proper instruments fitted when HRS was fitted!