Trufit Mainspring - Elgin 5550 for 15/0s movements - Steel

Trufit Mainspring for Elgin 15/0s movements, Factory No. 5550, steel, new in envelope. 15/0s are usually grades 536, 537, 538, 557, 558, 559, 623, 624 & 626. B&C Trufit mainsprings were distributed by Barton & Chase of Philadelphia but I'm pretty sure they are made in Switzerland. See image for precise characteristics. 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.