Need a Bobbin Winder Tension Disc for my machine?

I have lost the top part of my Bobbin Winder Tension Disc for my Singer Stylist Special Zig-Zag Sewing Machine Model 518. I went to put some white thread on a bobbin today and noticed the top part of the Bobbin Winder Tension Disc was not there, the part with the flat head screw slot on top.