1. top clamp or side clamp? Greist made both styles, side clamp fits Singer and other newer machines, top clamp will usually fit one specific mnfg machine. There are a few machines that take the same feet - and they are usually the less common ones.
2. Did it come with any plates, if so that helps a lot to determine what machine a set of top clamp will fit. I think I've come up with about 8 unique feet styles, and Standard is one of the less common, White, National and Free are the three most common of the top clamp.
I have the specs on all the different feet I have - so given those I can probably tell you exactly what machine your set will fit.