USR Mainspring for South Bend 0s No. 22528 - Steel

USR Mainspring for South Bend 0s, Factory No. 22528, steel, in frail envelope. I know it says 2528 but the "newer" factory number is 22528, and honestly these are pretty hard to find as South Bend didn't make that many 0s watches. I likely have several of these so the one you get may differ slightly from the images. I check these older mainsprings carefully before I list them all watch parts are unconditionally guaranteed. Images do an adequate job of representing the item.