As  in the other posts - the Series 2/2a was produced over a period of about 14 years. During this time there were dozens of different body styles and models produced on two wheelbases, and the company followed a policy of continuous product improvement. 
The only specific difference between the 2 and 2a was the introduction of the 2.25 diesel and the new chassis numbering system with suffixes. Petrol engined 88s built in 1958 (mostly) were fitted with the Series 1 2.0l engine. These are quite rare with this engine still installed and are becoming quite collectible.
In addition, the vehicles were built in several different places, from our perspective most importantly in Solihull and Sydney. 
For most changes, these happened not at a specific chassis number, but when the stock of old parts ran out - and it would not be unusual for batches of parts to overlap, particularly if, for example comparing Sydney and Solihull manufacture. 
Another factor was the change from BS to unified threads. This started with the introduction of the Series 2, but was still not complete at the end of Series production in the 1980s. In most cases the change is just listed in the parts book as "alternatives". And many original BS parts will have been replaced with Unified thread replacements. This will be particularly the case with bits that wear such as shackle pins. No metric threads were used anywhere on 2/2a Landrovers except for very late wheelstuds and nuts, but again, replacements may be. (Ignoring the fact that BA threads are actually metric!)
As local content increased, parts were likely to become different or to change at quite different dates.
Despite this, there are some significant changes that can be dated. Probably the most significant was the electrical system update between chassis number suffix 'C' and 'D'. This involved changing from positive to negative earth, and because it also involved changing from separate wiper motors to cable operation, it involved bulkhead changes. It also meant changes to fuel gauge and sender, instruments, switches etc.
Changes that did not come at specific chassis numbers or dates but can sometimes be listed as early or late include:-
Steering swivel spring loaded cones to railko bush - early
Steering swivel arms top to bottom - early
Vent screw to lever - early but not a 2/2a marker
Wheel nuts 9/16"BSF double sided to 9/16"BSF single sided but 27mm hex - late
Wheel studs 9/16"BSF to M16 - very late
Headlights in guards - nominally 1969, but did not change at the same time everywhere
Bonnet hinges 4 bolt to 3 bolt - late
Vent hinges bolted on to welded on hinges - fairly early 2a but not a 2/2a marker
Crankcase positive ventilation. Fitted to meet Australian rules mid sixties, but often removed.
Differences that have no useful date information include:-
Optional equipment - this includes blinkers, mirrors, trim, bonnet type, spare wheel carrier.
Tail and parking light changes 
Headlight changes
Carburetter - Solex was replaced by Zenith but the change date is unclear, and many will have been changed both ways.
Hope this helps
John
				
			 
			
		 
			
				
			
			
				John
JDNSW
1986 110 County 3.9 diesel
1970 2a 109 2.25 petrol
			
			
		 
	
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