Care should be taken not to try and remove these Wet/Dry knob caps by simply pulling, this could result in damage to the potentiometer.
With excessive use, if your knob becomes loose or needs the alignment adjusted, removing the plastic cover at the top give you access to the nut. (see attached photos)
If you need to order a replacement, please contact your dealer or local distributor
Part no - Description
AJ5613 - KNOB CAP BK-WH LINE 21MM