Singer "inherited" the back clamp from Wheeler Wilson. I have read that the rotary system they developed for the 66 used some of the patents they bought from WW. the exact attachment type is not the same, but similar.
The Singer D9 is the same machine as the W9 - so that is where Singer used up those supplies. Though they ended up making more feet, there is a difference and the Singer ones are made with slightly thinner metal so they have a metal slip added to the attachment end to make them the same width as the original WW where they clamp into the machine.
They early Chainstitch machines also have a type of back clamp foot - though it's a bit different. Singer went to something different later, but I have an early 1900's machine that takes the older style foot.
Most of the feet for the W9 are solid, Most of the glass ones I've seen are for the W8 - they look very similar, and it's likely the early W9's had the glass inserts before they went to the solid feet. I had a really neat set I sold to SteveH back when I was not planning on getting a W8 - and now I've got one so I've been watching to collect them again.
this is what I sent to SteveH
this is a W9 set - the attachment point is a bit different.
This is my favorite back clamp attachment set. It fit into a molded insert for the center drawer in a treadle cabinet.
