NuREZISTA Mainspring #651 for 21/0s Hamilton No. 7421 - Steel

NuREZISTA Mainspring #651 for 21/0s Hamilton Grade 750, 751, 756 & 757 Movements, Factory No. 7421, steel, new in envelope. See images for precise mainspring specifications. I specialize in hard to find watch parts and all watch parts are guaranteed. Images do a pretty good job of representing the item.