Hamilton Factory Mainspring for 16s 17-21j No. 718040 / 317 - Steel

Hamilton Factory Mainspring for 16s 17 - 21 Jewel Movements (such as the 974 & 992), Factory No. 718040 / 317, steel, new in frail envelope. This is a tough strength to find. 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 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.