Swartchild Black Shield Mainspring #4391½ for 16s Ingersoll Trenton No. 365 - Steel

Swartchild Black Shield Mainspring #4391½ for 16s Trenton and Ingersoll-Trenton Pocket Watches, Factory No. 365, steel, new in envelope. Could also very well fit in other watches 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 in hard to find watch parts and all watch parts are unconditionally guaranteed. Images do a pretty good job of representing the item.