Elgin DuraPower Factory Mainspring for 21/0s No. 6004 - Alloy

Elgin Factory DuraPower Mainspring for 21/0s movements, Grades 521, 523, 529, 533, 535, 541, 547, 620 and perhaps others, Factory No. 6004, alloy, new in envelope. Several different strengths were made for this mainspring and those I can supply can be selected from the drop-down list, with the most common strength for the mentioned grades being the .0036. See image for precise mainspring specifications. I specialize in hard to find watch parts and all watch parts are guaranteed. Images do a pretty good job of representing the item.