the earliest certain representation of a side ring on the quillons in art are on an engraving of Daniel Hopfer, maybe a type of grosses messer, believed to be Konrad von der Rosen. 1515 
(norman, The devopment of the hilt) 
 
My hypothesis is that the side ring is a development of the 8-shaped Katzbalger type of hilt. straight quillons were bent in eight-shaped ring quards; later this ring is closed and straight guillons are added. 
I suppose this all hapened in the first quarter of the 16thC. 
unfortunately I don't have any example of a 15thc sword with a side ring. 
 
fe see a sword in the painting of St Paul by Pier Francesco Sacchi 1523, it is clear to see that the side rings are still open.
		 
		
		
		
			
		
		
		
		
		
		
		
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