USR Mainspring for 12s Seth Thomas Pocket Watch No 2521 - Steel

USR Mainspring for 12s Seth Thomas pocket watch movements, Factory No. 2521, steel, new in envelope. Could also very well fit in other movements. I specialize in hard to find watch parts and all watch parts are unconditionally guaranteed. Ignore the paperclip stain on the outside of the envelope, otherwise images do a pretty good job of representing the item.