Swartchild Black Shield Mainspring for New England 0s Series ZZ #46 - Steel

Swartchild & Co Black Shield Mainspring No.4412H for New England Series ZZ movements, Factory #46, steel, new in frail envelope. See images for further details. I know then envelope says 0s, and this absolutely precisely fits a Series ZZ, but it will fit a Series Z and a perhaps a Series O as well, it's just a tiny bit longer than the stock mainspring was. I specialize in hard to find watch parts and all watch parts are unconditionally guaranteed. Images do a pretty good job of representing the item.