Hi SM and FW, yep, I tried Allen keys with no joy. The inner hole seems to be as round as a rissole.
I do have the neck I could practice on with the fret replacement, but with all the other symptoms this patient is presenting with I’m starting to think the owner should take it in to a proper guitar tech who knows what we are dealing with here. I would hate to make a mess of something as old as this, especially as it is not mine. From looking at these pictures I suspect we may be dealing with the traditional truss rod style.
I think I’ll swing by the local music shop today and see if they have anything to contribute.