Swartchild Black Shield Mainspring #44321 for Elgin 37s 8-Day Clock Factory No. 4457 - Steel

Swartchild Black Shield Mainspring #44321 for Elgin 37s 8-Day Clocks from the 1930's and 1940's (and perhaps beyond), Factory No. 4457, steel, new in frail envelope. Could also very well fit in other clocks as well. See images for further details. I likely have several of these so the one you get may differ slightly from the images. I specialize is hard to find watch parts and all watch parts are unconditionally guaranteed. Images do a suitable job of representing the item.