Abstract: An electrical instrument including a housing with a faceplate, a component for providing a variable electrical characteristic and a control shaft having one end coupled to the component and rotatable to vary the electrical characteristics thereof and an opposite end extending through the faceplate. Attached to the opposite end of the shaft is a coupling that is shaped and arranged so as to prevent relative rotational movement therebetween while allowing relative axial movement therebetween during removal of the coupling from the shaft. A control knob is attached to the coupling in a manner that allows both relative axial and rotational movement therebetween which movement can be restrained by activation of a locking means. An indication of the relative angular position of the knob is provided by a scale located adjacent thereto on the faceplate.