Swartchild Black Shield Mainspring #4374A for 16s South Bend No. 27528 - Steel

Swartchild & Co. Black Shield Mainspring No. 4374A for 16s South Bend 2nd Model movements, Factory No. 27528, 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.