Leyland Ska

I'm not convinced it was ever a 4mm. As I expected, 4mm first even touch the sides. The 5mm engages but isn't quite snug and I don't want to put any torque on it really. But it does look a little abused on really close inspection. Maybe it was a 5mm once. It's a B2A fretless with full inlays (looks fretted until you're up close), but I don't know its serial. Guess it is "uncivilised". Certainly the neck is like half a baseball bat.

I will definitely give it a proper go when I get home, thank you for the suggestion.

Hm. A 5mm hex is loose, sloppy fit in the socket. A 4mm will probably not even touch the sides. I definitely don't need to go smaller. Wondering if Hohner have ever used 13/64" or anything else exotic.

Trying hex wrenches (good quality ones), 5mm fits in the socket but is loose. 5.5mm and 7/32" don't fit. Could it be 13/64"? I can't reach a Vernier caliper into it, or get a decent imprint of it with Blu-Tac The hex socket looks in good shape, not rounded.