South Bend 0s Mainspring No. 12528 Made by Waltham - Steel

South Bend 0s Mainspring Factory No. 12528 made by Waltham, steel, new in very frail envelope. This is not the first time I've run across a mainspring that Waltham made for a competing maker and I don't get the sense that they did as much finishing on these as they did the ones they made for their watches. I specialize in hard to find watch parts and all watch parts are unconditionally guaranteed. Images do an adequate but perhaps incomplete job of representing the item.