The sword is in the book "Islamic arms and armor in the Metropolitan museum of art" page 180-181 
 
From the book (shortened) : Serpents and snakes are often found on iranian blades. Ultimatetly the image can be traced back to the biblical story of the staff of Moses, which became a serpent when thrown onto the ground.  
Many representations of the prophets sword are ingraved with snakes. 
It may also reflect some talismanic practice. 
 
There is a lot more information to be had about the sword in the book. Its a nice book, and not a bad purcase if you want the whole story.
		 
		
		
		
		
		
		
		
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