jonesynyc,
Welcome to the QB.
If that nylon disk is like the ones mentioned in some of my Kenmore manuals it goes at the base of the thread spool peg.
Everyone else just uses felt pads there.
You'll find a better source of parts at Sew-Classic {
http://www.shop.sew-classic.com/ }. She is great to do business with and much easier than eBay.
Joe