M201 - diode SORAL SI 195
The additional electrical unit mounted on the wing in the photograph is in fact a diode and is only found on M201 106SRs. It is secured to the wing by a mounting bracket and connected in line between the output from the regulator and the ammeter. Its purpose was to prevent damage to the regulator unit when the gun was fired. I guess the vibration could make the cut out contacts close in which case they would then be held closed by the current discharging from the battery into the dynamo causing damage to the regulator and contacts. Information about the unit is found in the army technical bulletin BT 632.
I am grateful to Barry England-Davis (M201 no. 16461) for these two photos of a unit that was removed from another 106SR.