Look at the knob. It has a place where a tiny piece of bowed spring tempered steel would slide into it. You aren't the first to loose the little rascal either.
3 options...
Make one from some other fastener using a dremel grinder.
Dill and tap the knob at the stem flat for set screw.
Buy a earlier knob that includes a set screw.
I'm looking for new knobs for my 74 swinger and I just keep finding stuff for 66-69 only screw on type. I would like to get the wiper and ligh knobs replaced any ideas
