I do not know of any 66 machine that is a "back clamper" except for the very early Singer 66's (maybe the early 99's as well?) before they went to low shank. As for the bakelite disc, we probably need a better lighted photo to be able to identify. Is there a center hole in the "thingy", is the hole threaded? From the photo view of the "thingy", I do not see any irregular lines or edges a cam follower would trace to sew decorative stitches.