Hamilton Factory Dynavar Mainspring for 21/0s Hamilton No. 717571 - Alloy

Hamilton Factory Dynavar Mainspring for 21/0s Movements, Grades 750 & 751, Catalog No. 717571, alloy, new in envelope. Made for Grades 750 & 751, and perhaps others in the upper 700 grade range. See image for precise characteristics. I likely have several of these so the one you get may differ slightly from the images. I specialize is hard to find watch parts and all watch parts are unconditionally guaranteed. Images do a pretty good job of representing the item.