We're in lockdown so I can't photograph mine but it's the same on this guitar only more pronounced - darker filler.
I'm pretty sure mine never wore Klusons as the filler is very clearly underneath the finish, which is checked over the top. To me, this indicates they had the necks pre-drilled waiting for Klusons then made the change to Fender Fs.
Does anyone know when the F tuners were first used, or was it a gradual thing? Are there any Electric XIIs out there with Klusons, for example?
