Hamilton Factory Mainspring for 18s No. 718011 / 15 - Steel

Hamilton Factory Mainspring for 18s Movements, Factory No. 718011 (or the old #15), steel, new in envelope. See images for precise mainspring specifications and be aware that people often mis-categorize Hamilton mainspring because the newer numbers are not as easy to follow (hence the chart), so you're safe to ignore just about anything someone has written on an envelope (the 974 is a 16s so this will not fit).  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 an adequate job of representing the item.