Suzuki 2 Strokes

What is the plastic knob on top of the center stem near the handlebars on my 71 T250? It does not seem to do anything. It doesn't even turn.

That'll be a steering damper. It's probably just rusted and stuck. Look below the bottom yoke/triple, and you will see the other end of the rod that the knob is joined to. It's just a crude friction type damper. Turning the knob should just tighten a couple of plates together under there, making the steering stiffer or looser as required.

This is what it looks like off the bike.