[RodB]

Ah, now it's clear. Yes, the ground terminal of the bakelite block is providing a ground for the tube sockets. Notice the center post of each socket is connected with a bare wire. This makes it convenient to connect tube pins to gnd without having to heat up the chassis with a soldering iron. Important note is to be sure the bolt in the bakelite block is very secure and has a star washer against the chassis. Schematics can be subtle; look at the transformer secondary providing 6.3v to the 7Y4 and pilot lamps. The 6.3v feed runs to the right and disappears into an arrow. This arrow connects to each arrow of the tube filaments and the other side of each filament is attached to ground (maybe the center post?). Full circle!