I know the later D1 pitman arms are different as I had to swap it when I fitted a D1 steering box to my County. 
From memory they are straight whereas the County ones are cranked. Splines are the same.
A quick look on ebay for some piccy's seems like the RRC are the same as the County/Defender so you should be good to fit the RRC one.
Getting it off could be a different challenge though  
 
Steve
				
			 
			
		 
			
				
			
			
				1985 County - Isuzu 4bd1 with HX30W turbo, LT95, 255/85-16 KM2's
1988 120 with rust and potential
1999 300tdi 130 single cab - "stock as bro"
2003 D2a Td5 - the boss's daily drive
			
			
		 
	
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