Swartchild Black Shield Mainspring for 6/0s Benrus 17j 1939 - Steel

Swartchild Black Shield Mainspring #441143 for 6/0s Benrus 17j "1939" wrist watch, steel, new in envelope. See images for precise mainspring specifications. A little puzzled by the "1939" as this is not related to a specific caliber, but given how Benrus watches were assembled and put into US made cases, I'm betting this will fit just about any early 17j 6/0s Benrus from that vintage. 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.