The S2 diesel would have been a 2.0. By the time it was due for overhaul, parts would have been difficult, as that engine was replaced by the 2.25 diesel. This may explain the change, and fitting a petrol would have been a lot cheaper and easier than fitting a 2.25 diesel. 
In my view the overdrive is not very useful on a lwb - I have fitted one, but almost never use it. It is coming off when I get round to it.
Note that the petrol and diesel (and six cylinder, but not the bell housing) gearboxes are identical, but there were a number of changes over the years, with the later the 2a box the tougher, in particular, the transfer case after about chassis suffix 'B' had a larger diameter intermediate shaft. Also gear ratios changed, but high range remained the same and the combination of first-low remained the same, but the main box ratios varied with the transfer low range changing to keep the overall lowest gear the same.
				
			 
			
		 
			
				
			
			
				John
JDNSW
1986 110 County 3.9 diesel
1970 2a 109 2.25 petrol
			
			
		 
	
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