Just checked a II & IIa Workshop Manual and as 1950landy said.....there is normally just power to the switch and then a feed to the main beam & dip beam. 
It's just a changeover switch that supplies power to either the dip beam circuit or main beam circuit.
Sounds like the PO has re-wired the headlight operation. Maybe when the dipswitch is operated it puts on the high beam as well as the dip beam. Who knows until you work out the circuit.
Colin
				
			 
			
		 
			
				
			
			
				'56 Series 1 with homemade welder
'65 Series IIa Dormobile
'70 SIIa GS
'76 SIII 88" (Isuzu C240)
'81 SIII FFR
'95 Defender Tanami
Motorcycles :-
Vincent Rapide, Panther M100, Norton BIG4, Electra & Navigator, Matchless G80C, Suzuki SV650
			
			
		 
	
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