William Mitchell Reversible Pen holder is a convenient pen holder with a nib unit that can be turned round and stored inside the pen holder to preserve the point from damage. The nib unit appears to be a moulded section but is in fact assembled from 3 parts (see photo). The parts can be separated and a new No.659 Crow Quill inserted in the pen. However, when the pen has been used it is likely that ink and paper have worked their way up the nib shaft and the nib itself has started to corrode.
It may help to soak the nib unit for 24 hours in washing up liquid or air brush cleaner to soften the debris round the nib shaft. Grip the nib unit and nib and twist the nib around the supporting pillar. Bear in mind that the nib unit parts are plastic and breakable. Since the nib needs replacing there is nothing to be lost by gripping the end of it with pliers. A twisting and rotating movement should enable the nib to be pulled out of its socket.
The supporting pin may remain in the socket but may need to be pulled out to enable the socket to be thoroughly cleaned.
Refit a new No.659 Crow Quill and your pen is as good as new. !!