If this is what I think it is, the Gearing Pad was fitted to the Mk.10/SEP2 Main Amplifier, and was accessible under a flap in the side.
Its main purpose was to alter the gearing ratios and feedback rates of the three main autopilot channels, pitch, roll and yaw, according to the required aerodynamic characteristics of the aircraft type it was fitted to. Each aircraft type would have its own specific pattern of links which altered the inductance dynamics of the magnetic amplifiers. AFAIR, it also enabled other attributes of the autopilot to be made "live" or not.
Thus, a standard piece of avionic equipment could be specified and ordered for many different aircraft types--good thinking, and a good example of early COTS.
Having written the above, I seem to recall it being referred to as "the Links Pad", so there is a chance that this example might be different!
Sorry m'lud, it's all a very long time ago...........