Could need some help/ guidance finding a locking nut for ST ferrari neck
I’ll ask if they still have spare parts for this model, but as these guitars were manufactured from 1985 - 1988 it’s not that likely. You may be better off looking for a generic part or picking up a used one on Ebay.
Service Dept. say they haven’t stocked parts for any ST Professional models since 2012. They recommend that you enquire at this forum:
http://www.musiker-board.de/userthreads-e-git/275107-hohner-professional-user-thread.html
Hope that helps
Steve