Off topic but I see no problem modding a vintage amp when it was already modded to begin with. Most of what you see out there has already been drilled or modded. Thats what I look for if I want to mod one. My 73 pcb had four holes already drilled so I had it modded.
My 73 Superlead original turret had one hole drilled on the back for a PPIMV earlier in it's life. I still consider this amp an amp that should not be modded