That is a cool looking amp. I would remove the tubes and measure the green wire to all the other power transformer wires. I guess my guess was wrong.I wonder what the wire is for.
Page 524, Figs 12.30A and 12.30B, but it is not quite like either one. It is one of a huge family of Paraphase drivers. The Patents and the RDH are bullplop; real designers who had to keep it cheap and keep their job did it as in your schematic.
Page 540, section 12.10.G.2, suggests heater CT to a small bias voltage, but does not suggest the output tube cathode. (I suggest that this would be "obvious" to anybody who had spend too much time poking a meter in a tube radio.)
Betcha it goes to the first stage cathode.
http://www.schematicheaven.com/bargainbin.htmScroll down to Gretch in the left column. They don't have your model, but the others have the same mix-n-match pieces.