Spiral, 
 
At first, I thought maybe you misunderstood me.  However, after rereading your post, I think you're probably right. 
 
I assumed that the author knew the difference between a knife and a dagger, so I was stuck looking for a double-edged blade.  I now think it's most likely that he makes no difference between them and the picture you linked is what I was looking for. 
 
Thanks!
		 
		
		
		
		
		
		
		
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