[Milsurplus] DAV-2 On Off Wiring

[arc5 at ix.netcom.com]

I think it's "cheap and easy" sidetone, plus a way 
to boost the carbon microphone voltage.
If you follow the circuit out, the mike voltage is the 
"C" voltage in series with the "A" voltage, giving 
6 volts for the carbon mike.  
Also, the mike voltage gets tapped to the 
headset, which has a potential difference between
the two batteries, but no path to ground.
So you get cheap sidetone.

One guy's theory,
Dave S.