USR Mainspring for Seth Thomas / Century 18s No. 729 - Steel

USR Mainspring for Seth Thomas & Century 18s, Factory No. 729, steel, in original but frail envelope. I likely have several of these so the one you get may differ slightly from the images. I check these older mainsprings pretty carefully before I list them all watch parts are unconditionally guaranteed. Images do an adequate job of representing the item.