Swartchild Black Shield Mainspring for 12s N.Y. Standard Model K No. 4284 - Steel

Swartchild & Co. Black Shield Mainspring #441001 for 12s N.Y. Standard Model K movements, Factory No. 4284, steel, new in envelope. See images for further details. I likely have several of these so the one you get may differ slightly from the images. I specialize in hard to find watch parts and all watch parts are unconditionally guaranteed. Images do a barely adequate job of representing the item.