This is a set of back clamp attachments for a Singer 66 - this set would be from the early 20's at the end of the back clamp machines.
This is the same set for the side clamp. The under braider and shirr plate are missing, but they are identical to the other set. The contemporary 15 takes the same feet, but the underbraider and shirr plates are larger to fit it's larger needle plate.
This an early set for a Singer 28K
