Stumbled on a cut-away for a precision multi turn pot.
There's a car that rides in the gap between the adjacent coils of the wire wound resistive element, I believe it houses or provides alignment for the wiper.
In other news, the Peavey Mark IV was put to test on a speaker load. After a minute or so of fantastically textured white noise it passed signal well. The floor noise diminished with use, isolating the cause of the noise is on my to do list.