Alistair, Armoured O.P. Carriers (armoured observation posts) were usually attached to the Artillery. they were fitted with radio gear and usually called in artillery fire on various targets. There were only 95 MkIs built . these were  Scouts that were fitted for radio. Most of these were lost at Dunkirk. 
The MkII was a universal, again with the observation shutter, also extensively kitted out with radio gear. The mkIIIw carried an extention ladder and a pole for dealing with telephone cable. 
 If I recall correctly all models carried reels of telephone cable mounted front or rear or both, able to be laid while driving. 
I have not seen any info, (which just means i haven't looked for it) that refers to  AOPs working with a Mortar platoon. 
I hope that helps?
		 
		
		
		
		
		
		
			
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				Bluebell 
 
Carrier Armoured O.P. No1 Mk3 W.  T84991 
Carrier Bren No2.Mk.I. NewZealand Railways. NZR.6. 
Dodge WC55. 37mm Gun Motor Carriage M6 
Jeep Mb #135668 
So many questions....
			 
		
		
		
		
		
	
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