1983 Precision Elite II truss rod nut issues

[Z Axeman]

I own a 1983 Elite II that I encounter problems with to access and insert an allen wrench for adjustment into the truss rod nut. None of my regular size truss rod tools (i.e. Fender 3/16") will fit.
The nut looks like it shifted in the access hole and appears to be bigger than the access hole, so it might be impossible to extract it, should it become necessary. Previous owner seems to have had already issues with it already, and some of the rosewood north of the nut is damaged. (please see pics attached).
Allen hex looks ok in pictures, but wrench won't grab.

My questions,
what size allen wrench is this supposed to be?
Are there known issues with these basses from back then regarding truss rod adjustment?

 

[Turnaround]

The truss rod nut is not removable on this bass without removing (drilling) the walnut plug at the truss rod end. The messed up end of the fretboard is not a functionality issue, just a cosmetic one. Get it fixed if it bothers you. The truss rod nut looks fine - you just need to find the right size allen key to fit it.

 

[Z Axeman]

The truss rod nut is not removable on this bass without removing (drilling) the walnut plug at the truss rod end. The messed up end of the fretboard is not a functionality issue, just a cosmetic one. Get it fixed if it bothers you. The truss rod nut looks fine - you just need to find the right size allen key to fit it.

Thanks for the input, that answers one of my questions. The cosmetic aspect is unfortunate, but not a pressing issue to attend.
As I have difficulties finding the right size allen (I tried to get a hold of that hex with multiple sizes, domestic and metric without messing it up!), what size should it be ?