Swartchild Black Shield Mainspring for Hamilton 6/0s No. 1721 - Steel

Swartchild & Co Black Shield Mainspring 4372F for Hamilton 6/0s movements and perhaps others as well, Factory No. 1721, steel, new in envelope. Swartchild Mainspring Catalog (which I sell) lists this as a Dennison 4 x 8.5. See image 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.