Swartchild Black Shield Mainspring #4374 for 16s South Bend No. 217528 - Steel

Swartchild Black Shield Mainspring No. 4374 for 16s South Bend 1st Model movements, Factory No. 217528, steel, new in envelope. See images for further details and precise mainspring specifications. 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.