Although I have not seen the actual ribber yet, I was excited to discover that super vintage Silver Reed SK-7 knitting machine can be accessorized with a ribber. Its manual/description is in the manual for the Silver Reed SK-7 main bed.

The SR-7 ribber looks similar to the Silver Reed SR-200 ribbing attachment, which makes me suspect that SR-7 might fit SK8 main bed (especially because SK8 and SK7 are very similar machines).
Stay tuned as I am scouting for more info and more pics.
