Swartchild Black Shield Clock OPEN LOOP Mainspring 3.00 x .25mm x 19½"

Swartchild & Co Clock Mainspring, OPEN LOOP End, Dennison 2/17 x .0098 x 19½, from the 1950's, steel, new in envelope. See images for further details. Envelope suggests spring is suitable for a Westclox Travel clock but I've had no success identifying which one. It is also suitable for several European travel clocks. 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