[R-390] Need info on R-390A type or variation

[Phil]

I recently lucked into an R-390A at a hamfest less meters and
a few tubes that is dirty but restorable.  However, it is missing
the front panel tag and black spray paint has been added to the
back panel to cover any printed info.

There is a long rectangular cover about 1 1/2 inches wide that
runs across the back panel and covers up the barrier strips as well
as the power connections.  There is one larger 3-pin amphenol
military style connector in the middle labelled "power" and two
smaller 2-pin mil style connectors near each end that are not
labelled but I'm guessing might be for the two audio output
channels.

Can anyone tell me what this version was used for?

thanks, Phil
W5BVB