Swartchild Black Shield Clock Mainspring 4.0 x .30 mm x 28½" Ref 40-30 B 72

Swartchild & Co Black Shield Clock Mainspring, OPEN LOOP End, GR Ref. 40 - 30 B 72, steel, new in envelope. See images for precise mainspring specifications. I specialize in hard to find watch & clock parts and all parts are unconditionally guaranteed. Images do a pretty good job of representing the item