Hi Puckdropper,
I did find that link to the PDF referred to in quite a few places. I never thought to write to them and ask if they still have it

I also found an old parts diagram but that hole is not drawn in.

I have just been out to measure the hole and it is 5mm dia by about 16mm deep.
I can feel some notches in the shaft if I put a piece of wire down there.

Now that I look at the hole closely I see a bur in the direction that would form if an undersized rod was used to hold the shaft while the Center was removed.

I guess I'll find / make something the right size and have a go.

If the worst happens I'll just dissemble the headstock. If nothing else I'll see the condition of the "notches" and what the bearings look like.

Thanks for your input and I'll post the results here.

David.