Swartchild Black Shield Mainspring for 12s N.Y. Standard No. 4144 - Steel

Swartchild & Co. Black Shield Mainspring #21233 for 12s N.Y. Standard Model B movements, Factory No. 4144, 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 pretty good job of representing the item.