Joe, thanks, you're right -- the white tab is a stopper.
I did move the 2 knobs for setting and locking in the stitch width and the machine does not respond.
I believe the cams are the right ones, they fit size-wise and also in the guide that sticks up, and the black arm that traces the edge of the cam does reach the edge. That mechanism works, but does not translate into movement of the "swing arm". Somewhere is a breakdown in communication, maybe as minor as a spring that needs to connect to something.
I will dismantle some of the mechanism tomorrow. I did read of broken cam gears somewhere, even a metal one.... so I'll look for that.
Foufymaus, if you have time, can you take a photo or 2 of your machine to compare? I did adjust the stitch width to max without any result. And using the cams has no effect either, all I get is a straight stitch. (BTW, I did see a set like mine on eBay.) Thank you for your suggestions.