Dimmer Switch - it was the "SARBE / Autolycus" switch, which IIRC was checked in the "Tac Checks Outbound" - I thought I'd taught you better than that!
The Autolycus equipment had definitely been removed from the MR1 post 203 returning from Malta, it was replaced by the 3rd Sonics set, giving us dry men a chance to monitor cold active buoys........