Ive very much enjoyed your replies to this piece and I thank you. 
The Dagger is a total of 55cm long, with a Blade 41.5cm.  The maximum thickness of the Blade at the Forte is 8mm. 
With regards to the Monogram I would say that it is RN, with No.43 below. 
The serrations on the Blade start just under 7cm from the tip and runs for a length 4cm, finishing 2.7cm from the tip. 
The channel in between the grooves is about half the thickness of the Blade deep. 
I do agree that is possibly Naval.
		 
		
		
		
		
		
		
		
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