Based on British practice: 
Position of the small red square indicates the battery - starting top right (for 1 Bty) and moving clockwise - so this is No3 Battery. 
 
Then it starts to be a case of understanding the regiment's dialect - the norm is: - 1 Bty has A, B & C troops; 2 Bty D, E & F and so on - but this scheme was often varied - B4 should represent No4 gun of B troop - Haven't a clue what the 1 represents though. 
 
Noel
		 
		
		
		
		
		
		
		
		
	
	 |