Hi and thanks for your replies! After further investigating into my serial number, I found that my machine is actually a model 127, not a 66 as I'd thought.
irishrose: the foot clamps from the side and that is where the little screw attaches as well. So maybe the generic side clamp walking foot will fit.
barbgooch: Not sure what is a low or high shank, but I measured the distance from the top of the foot that's on it to the base, and it measures about 3/4 to 7/8 of an inch. (This is the "regular" foot that I always used, with the right side measuring about 1/4 inch and shorter in length than the left side.) Is that low shank? But the attachment is from the side, so I guess those wouldn't work after all. Thanks so much for offering, though!