Yeah it is an early one, before the Rio takeover... (It came with a Empeg stick). So I will try to contact them. Thanks for the advice!

Alternatively, if you don't want to wait... put a tiny (pinhead-sized) drop of glue on the shaft and let it dry. This will create a small "bump" that, when you put the knob back on, will keep the knob from slipping. (DON'T put the knob back on until after the glue has dried!)

tanstaafl.