South Bend Factory Mainspring for 16s No. 27528 - Steel

South Bend Factory Mainspring for 16s 2nd & 3rd model movements, Factory No. 27528, steel, new in frail envelope. Checked it to verify it's a Dennison 19 x 5. I specialize in hard to find watch parts and all watch parts are unconditionally guaranteed. Images do an adequate job of representing the item.