Speedometer Trip Milage adjustment knob

Here's hoping someone can offer me some advice.

The trip milage adjustment knob came off the speedo in my TR3a.  I noticed that it has two different sized holes on the tube on the knob and they line up with a hole on the shaft out of the speedometer.  It seems like a small length of piano wire could be fed throughthe holes and be functional.  However, since the holes on the tube on the knob are differently sized there must be a specific piece that should be used. 

Can anyone advise me?  Many thanks,

Tim