[ARC5] Australian AWA "Radiotron" 807 Valve

[David Stinson]

I'm working on a neat RAAF AT5 transmitter.
One of the tubes is an Australian-made AWA Radiotron
type 807.  Engraved on the brown bakelite base
is a capital "D" followed by an up-pointing arrow 
and another capital "D."   The arrow is between the 
filament pins and the two "D"s align with the fil pins.
Anyone know the significance of this marking?
Thanks,
Dave S.