The plate with the rounded lip fits the 66/99 The other fits any of the side clamp machines, though it came standard with the 101. Though the plate that fits the 101 is slightly different.
Singer quickly went to a longer darner foot - and if you ever use this one, it's pretty evident why. It replaces the needle clamp, and you can tell that the needle goes quite a way into the fabric before the foot even touches it. It's better than nothing - which is what singer recomended for darning before they came out with this foot.
BTW - these are not cheaper, common parts. If you get the plate for under $10 you are doing good, and the foot can go for more. Best way to get them is watch for misc lots that have them in, and hope no one else notices.
The listing this picture came from is at a good price - I'd suggest trying to buy it if it does not go much higher if you really want this set.
Also - the listing is a bit misleading, while the foot fits all listed, the plate will not fit anything but the 66/99 machines.