There is an INSIDE Foot and an OUTSIDE RIGHT TOE Foot. Both feet have a 1/4" (6.4mm) GROOVE cut into their bottoms
This size 1/4" (6MM) is the most popular size used for all kinds of Welting Operations on Furniture and Upholstery.
INSIDE FOOT #067-22-069-3 (And set screw ~#992-00-687-0)
OUTSIDE FOOT #067-22-077-0 (And set screw ~#893-06-716-0) .