1974 Fender Twin Reverb (Master Volume) Circuit Question

[reginom72]

I purchased this amp new in mid 1974, loved it and used it until 1981 when I got a Mesa MKIIB. It did have the 2-12 JBL's factory installed.

One peculiar thing about this amp is that it had a beautiful "cleanish" Marshall tone very much like what you hear on the Allman Bros Live at the Fillmore East album, but only when the master was on "3".

From what I recall, I set the volume on "7" with the pull boost (MV knob) engaged and with the MV on 3, and there it was.

Oddly enough, anything above or below "3" on the Master Volume would revert back to a "clean Twin Reverb tone".

Is anyone familiar with this amp and in particular this phenomenon? 

I would love to know what caused this to happen only at "3" on the MV, as I'm having a SFTR head clone built for me and would like for that tone to be available at any volume.