Why would a tube radio leave the 6.3V heater tap on the transformer disconnected?
That looks like it might be an arrow-head on the end of that transformer tap, which I would be inclined to interpret as something along the lines of "we all know where this goes and we don't want to clutter up the schematic by drawing it in everywhere."  

So in this instance, I believe that you're intended to connect that tap on the transformer to all of the heater filaments.