Unless there is something funny going on, the two terminal tank unit (529969) is for a diesel and the one terminal (519838) is for a petrol. The extra terminal is for the fuel warning light.
But note that quite apart from the number of terminals, there are two types of sender used on Series 2a Landrovers. One type was used from Series 1 up to the change to negative earth (and other changes) in 1967 (numbers above), and the other from then until the end of Series 3 production (555844,555845). The two are not interchangeable unless the gauge is changed as well, and the later gauge/sender requires the use of a voltage stabiliser. (different senders on the rear mounted station wagon tank)
So, your old sender is a diesel one (whether your engine is diesel or not), and the new sender should work if you connect the green/blue wire to it - provided the sender is the correct pre-1967 one, and your fuel gauge is original. This leaves you without the warning light.
John
				
			 
			
		 
			
				
			
			
				John
JDNSW
1986 110 County 3.9 diesel
1970 2a 109 2.25 petrol
			
			
		 
	
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