6 volt cigarette lighter, socket tapped right off the PT, is my guess. 6 volt cars had cigarette lighters you know...https://www.autozone.com/lighters/cigarette-lighter-and-accessories/p/dorman-56411-cigarette-lighter-assembly/439342_0_0
Lighter part pretty cheap and easy. Even the extra hole in the faceplate isn't so bad (probably doesn't make "cents" to do this to an actual Bassman).
The 12VDC power supply is part I'm struggling to figure. Maybe just a wall wart and a barrel connector on the back of the amp? Use a 2.5mm so you don't mistake it for a pedal power supply?