[R-390] 390a audio

[Don Reaves W5OR]

A few years back, I bought a tall rack cabinet surplus from the FAA.  Just
this week I stumbled across a piece of gear that came out of that cabinet.
At the time I paid it little heed, but now it looks more interesting for it
is a multi-channel 600 ohm mixer, rack mounted in a 2U box.  Marked Audio
Mixer - Amplifier Assy Eight Channel Model No. MAA-8/600 made by G.R.M Corp
in Medford NJ.  My question is does anyone know about this unit or the
company that made it.
Some of the channels are marked as Flight Data 1, FD2, preflight, RDO.  

This might make an ideal audio mixer for all the 600 ohm output receivers
that need to be tamed here.  No mods necessary to the R-390s.  

Each channel has way too many input/output pins (24) to casually reverse
engineer.

Anyone?