Thanks for your reply, NapaJohn
For now, I've ordered a new low shank foot from this website,
http://www.sewingpartsonline.com/sew...sser-feet.aspx
and they also steered me to that same snap-on adaptor, and they also sell it with a full set of snap-on feet. It isn't the piece I'm looking for though, as I don't think my current loose feet would work as far as snapping onto that device.
My broken piece is a Sears Clamping Shank, Part #37170, and they no longer sell it.
My search will continue, though. I've seen the piece I need sold on Ebay, but it's always been part of a bigger accessory set of Cams, buttonhole attachments, etc. -- all the things I already have, and I didn't want to spend what the collections were being offered for. I'll keep watching Ebay and cross my fingers that I come across one at a reasonable asking price there.
Thanks again for the kind response!
Mary Ann