Kitsy...it looks to me like it's a spring that goes inside of a clamp, like a clip on lamp, or something similar.
I know it won't help you now with your bobbin winder, but in the future take pics of your projects as you're taking them apart, that way if you get lost, you can look at the pics and see how it goes back together!
Also..use masking tape to tape the screws to the hole they go in, so when it's time, you don't have to guess which scew gows where.