This is a really neat set! I've seen pictures of the main foot on a W8 - the shirr plate does line up correctly to be used with the W9. Don't know if the feet would fit as mine needs a bit more tri-flow before the screw will come off.
This is the attachment foot - with the glass insert (and yes, it is glass!)
And attached to one of the hemmers.
I bought the attachments in one auction, and a bunch of misc items along with two bobbins was in a second, and I won that one too - the glass part was in with the second group. When I see unique items, I like to check out other listings by the same seller. Though in this case I didn't realize what that bit of glass was until I had it in in my hand.