Well, your Master Volume mod 'could be made to appear' to have used the voltage selector hole. Meaning, the amp could be fitted with a Voltage Selector switch (working or dummy) to give the no 'extra' holes look if & when restored. Being I don't see the 2 little screw holes that mounted & held the voltage selector in place that should be left visible.. I'd guess that by 1975 the Voltage Selector was forgone & pre-set for our American market as seen on the back of my 1976 JMP 50 watt Master Volume. (1976 50 watt Master Volume rear panel without voltage selector) (stock JMP photo with voltage selector) (your amp below) Now I don't have your amp in front of me & I might be wrong but it appears the pre-amp signal is jumpered over to the rear panel Master Volume mod. (Your amp with Master Volume mod wire highlighted) So it's also my guess that your cleanest sound is going to be found keeping the front panel volumes on or below 4 (All my 4 hole 'non-master volume' JMP's start to get power tube driven tones after passing 4 on the Volumes) & then lowering / adjusting the rear panel Master volume to taste. The Volume on 4 is usually pretty loud on these old JMP's.