[ARC5] Ideas on Japanese circuit ?

[Hubert Miller]

I have been trying to figure out the schematic and connector pinouts for a Japanse I J A Type 99-1 
radio set. Here's something that puzzles me. Maybe you have an idea why done this way?

Rec dynamotor unit has 2 separate output fields. One has -B and +B to receiver. Other has one side
grounded, the + side goes like this:

LV dynamotor =>  connector => cable => receiver connector #4 => ? drops off LV for tube filaments ? and pilot
lamp and jumpers to receiver connector #6 => cable to transmitter connector #6 => keying relay and
microphone current. 
I cannot see how the tube filaments are supplied as the filament connections are not shown and the Japanese
text characters are too small to see clearly.  
WHY would the keying relay and mic be powered off the dynamotor and not the 6 volts main supply? 
Maybe this is so you could have the input power polarity either neg or pos ground? The dynamotor primary is
NOT connected to ground, both leads floating, so it could be powered either way. 
What think?
-Hue Miller