Unfortunately shaping revealed that the Black walnut wasn't dried very well and has split. More fortunately it isn't structural at all and I have solution that I've been meaning to try anyway.
Shaping has also revealed (note the passive description, obviously it wasn't my mistake) that the tuners are too short for the slots.
My two solutions are above. Brass dust and thin glue to fill the cracks, a little like the gold in Japanese Wabi Sabi pottery, and modified ferrules (modified with black milliput) to stabilise and bridge the gap in the slots.
Sanding the ferrules using drill and bolt as a mini lathe.
Ferrules almost ready.
And here is my first attempt at a brass tailpiece (not quite finished of course). Far easier to cut than I expected. If it doesn't quite work I'm much less worried about making another.