I was playing around with some Sifam collet knobs the other day and i unscrewed the 6.35mm brass collett too far and it dropped out.
I though - hang on...
I have a load of 6mm brass collett knobs as well
So I took the brass collett out of those as well.
Now I am a bit thick when it comes to DIY bits'n'bobs - but I just had a blinding flash of light and swapped them over
Yes - they are exactly the same outer diameter to fit into the knobs - but the inner diameter is either 6mm, 6.35mm etc...
So I can now buy readily available 6.35mm knobs and swap out the colletts from some "wrong colour" or "wrong size" knobs
SO you could buy 6.35mm pots - but just change the collett over from another knob
May or may not help.. but it was really a DUH moment for me.