They are shirring plates, they work with the ruffler for doing gathering when you aren't right at the edge. You start by removing the shirr plate that comes attached, and then putting this one on the bed of the machine.
Since these mount to the bed of the machine, they are machine specific. I found a list on Needlebar about what machines Greist made sets for, but I don't know if the list is complete or not.
National Rotary - fits my Two Spool
National VS/Other
Free VS - fits my Free "Long Bobbin Rotary"
Free/New Home Rotary
New Home VS (old) - I want to get one of these . . .
Singer (side clamp - the only style I've seen would fit VS machines). I have plenty of these
White Rotary - Fits my new White with the molded head.
White VS (says one style has bed mount hemmers) - I have the foot mount hemmers - fits my New Queen