It's a twin to my Kenmore 117.740 also made in Western Germany. I use it's "kissing cousin" my avatar daily so I can help you figure out your machine if you want. Just send me a PM. The knob you are curious about is definitely to drop the feed dogs. When I got my machine, it was also stuck. I oiled the heck out of it and ended up giving it a tap with a hammer - very very lightly!!
I've also attached pics of the cams. I have knee controls on both of my machines and really like them.[ATTACH=CONFIG]557591[/ATTACH]
[ATTACH=CONFIG]557590[/ATTACH]