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) .