"Brown Eye" / "Hairy Brown Eye" - No info on the actual mods done to the Marshall circuit; apparently this isn't a "public" mod but one offered by some amp tech named Dave.
It definitely does have a Master Vol; whether PPIV or not can't tell without looking at the guts.
You can see here:
Notice the "Master" and "Gain" on the right side of the controls... normally on the stock circuit those 2 knobs are Treble Channel Volume "Volume I" and Normal Channel Volume "Volume II". One of the
very key aspects of that glorious, fat crunchy 4-input Marshall tone is the
lack of a Master Volume; if you want more gain, you have to turn it
up to get those power t00bs cookin' (that's where the magic tone happens).
So, at least he's being smart and not drilling holes in the panels (front or back) for the Master Vol - this very intrusive mod (drilling holes in the panels for Master Vol knobs) was typically (and sadly) done to so many vintage 4-input Marshalls over the decades... mostly in search of the "Brown Sound" (EVH). Some were even cutting the chassis out to add an extra preamp t00b cuz they read some quote about Eddie's tech Jose A. doing so to Eddie's amp (which was never the case).
Anyhoo, a typical MV circuit can cause the treble in the tone to get thin and buzzy. I think at best you can get it to sound good, but it's always going to sound (at best) somewhat thinner and buzzier than the typical old-school 4-input Marshall circuit.