[Arran]

Pete;
  There was an older version of a 6SF5 called a 6F5, but it has a grid cap, and the pinout is different, a 6J5 is similar again but I think that the trans conductance is different, why Electrohome chose to use two different single ended triode tubes right next to each other, only they could answer. A lot of sets used a 6F5 or a 6K5 along with a 6H6 to perform the same job as a 6Q7 or 6R7 would in another set, but it looks like D.E chose to wire up a triode as a diode in your set.
  Yes, they call those small batteries bias cells, Electrohome must have had a liking for them in the 1940-41 model year as the Phonola console I have also has a pair of them. I wonder if they may have hired on some engineers from Rogers or Canadian Marconi as those companies were infamous for the use of bias cells, from about 1936-37 up until the war? I think that it must have been some sort of patent royalty avoidance scheme, possibly to RCA or Hazeltine Research, as all three of those companies used cathode bias/self bias, or fixed bias via the power supply, for their tubes before adopting those silly bias cells.
  The company that developed and market these was P.R Mallory, a battery and radio parts manufacturer, the cells are carbon zinc and put out about 1.4 volts, at almost no current, there were various advantages they cited in their marketing but I think that they were tenuous at best, the only one I could understand is that it's a source of grid bias with zero AC ripple. In many cases you can rewire the tube to self bias, but modern watch batteries are not difficult, nor expensive to buy, though the polarity is opposite that of the old bias cells.
Regards
Arran