Hamilton Factory Mainspring No. 1721 or 1921 for 6/0s - Steel

Hamilton Factory Mainspring No. 1721 or 1921 for 6/0s 17j movements and perhaps others, steel, new in envelope. See image for precise characteristics. 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 so I usually have whatever you might need and all watch parts are unconditionally guaranteed. Images do a pretty good job of representing the item.