And for the missing piece is this, I recovered a little bit when I ripped it out: The OA2 tubes, along with the 1st and 3rd leads of the pot, are in parallel with the 30MF caps, and the cathode of the error amp tube is connected to the center tap of the pot.
And BTW, taking apart the PSU I found the schematic; notice the date…. it's literally as old as my mother lol
The pass tubes are 1619 (or VT164), the error amp is 6sj7, the rectifier for +B is 5v4g, and the 2nd one is 6x4.