The spring is a flat steel indent spring that holds the wheel in its setting. It is often missing. I had my gunsmith make me some a few years ago but have used them up over the years.
I don't have a picture and have very little camera skills, but if you turn the body of the sight over, you will see two little holes in the front. Using spring steel, cut a piece about 1/16" wide. Put a little "V" bend in the center and have it long enough to stick out and catch the indents in the wheel. The gunsmith who made mine played around a little to get the length right. If you have some spring steel stock, they are not too hard to make................tom