Hamilton Factory Dynavar Mainspring for 21/0s Hamilton No. 717570 - Alloy

Hamilton Factory Dynavar Mainspring for 21/0s Movements, Catalog No. 717570, alloy, new in envelope. Made for Grades 75, 77, 750, 751, 756, 757, 761, 780 and perhaps others. See image for precise characteristics. 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.