Sandsteel Mainspring for 12s N.Y. Standard No. 4144 - Steel

Sandsteel Mainspring for 12s N.Y. Standard Model B movements, Factory No. 4144, steel, new in envelope. See images for further details. I know the envelope says 4282 but that's a typo as the factory number for this mainspring is 4284. 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.