Hi David, 
 
 
They seems strange indeed and troubled me too at first; somehow 'out of period' (or rather of what we expect to be 'period'). 
 
On the other hand, quadratic bolt heads were in use since at least the 15th c. as a small Gothic alcove gun, preserved in virtually 'untouched condition' in my collection, shows. 
 
Similar devices are depicted in Philip Mönch's Kriegsbuch (Book of War Techniqes), 1496, fol. 32r. 
 
 
Best, 
Michael
		 
		
		
		
			
		
		
		
		
		
		
		
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