Swartchild Black Shield Mainspring #4347 for Hampden 16s No. 3696 - Steel

Swartchild & Co Black Shield Mainspring #4347 for Hampden 16s ¾ plate movements, Factory No. 3696, steel, new in envelope. Swartchild Mainspring Catalog (which I sell) lists this as a Dennison 18 x 3. See images for further details. 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.