TTM
I'm not a auto electrician, But from experience, some of the bulbs illuminating the Instruments and behind the dash on BEX333 are either a tight pushfit into
rear of each gauge or are on small metal strips secured by the knurled nut to the bracket to the rear other gauges to illuminate the circumference..
Some Smiths Gauges take larger bulbs. Some bulb holders take bayonet bulbs where as some take screw threaded bulbs .... when the change 1950s /1960s ??
Have you tried pinching / reducing slightly the diameter of the reciving femail socket / bulb holder then pushing the bulb back in ?
regards the switch on your dashboard , could this be just to fash your headlights, My 57 LHD Ace has dip switch on the floor to the left of clutch pedal, not sure if this is the same on RHD.
Not sure if this helps, Hopefully someone with more knowledge can throw some light on the subject ... Excuse the Pun, not intended.
Keith