Magnetic Compensation Module

None, other than Air Ministry logo and mounting instructions (AFT).

The gyromagnetic compass is a horizontal gyro slaved to magnetic north by a magnetic field detector (flux valve) usually located in the wing tips of the aircraft away from metal structure. The short term accuracy of the gyro combined with the long term accuracy of the magnetic compass provides a stable and accurate heading reference.
The Gyromagnetic Compass generally has two operational modes:
Magnetic slaved information (also revered to as “SLAVE” mode) – this mode is used when magnetic heading references are reliable. In this mode, the directional gyro is slaved to the magnetic flux detector, which supplies long-term magnetic reference to correct the apparent drift of the gyro.In order to ensure correct indication of the directional gyro indicator it is necessary to align it with the magnetic compass at regular intervals, approximately every 10 to 15 minutes. On some modern aircraft this alignment is performed automatically by the control system.
Directional gyro (also referred to as “FREE” mode) – this mode is used when magnetic heading references are not reliable (e.g. in polar regions). In this mode, the system furnishes an inertial heading reference, with corrections introduced manually to offset earth rate and other errors.