I'm calling this a "black" message box because it has no ID markings of any kind.  In the Radio command menu is mentioned "DMD" so this may be called a Digital Message Device.  Some other DMDs are:
PSC-2 Digital Message Device (DMD)
PSG-2 TM 11-7440-281
PSG-5 TM 11-7025-244
PSG-9
OA-8990/P
AN/UAS-12 ?
Nokia PARSA

This unit was designed to be mounted on a radio using the four captured screws on a hole pattern that's 8-1/8" x 3-1/2".

If you know of a radio with this hole pattern or have any info on this unit please let me know.

When it's turned on (and after the LCD contrast is adjusted using <shift> and the up or down arrows) the LCD displays:

Radio status menu   <<  ^  v  to  scroll  >>                               << ENTR to select>>	v     << Radio Status Menu  >> Radio Status Summary: No response from radio Activity Monitor: Epoch time:, Msg status: Past Epoch Status: No response from radio Link Quality Check: Is radio in NORM? YES: No response from radio Antenna Tune Quality: No response from radio
Interoperable mode <<  ^  v  to  scroll  >>                               << ENTR to select>>	Prepare message for Tx
Radio command menu <<  ^  v  to  scroll  >>                               << ENTR to select>>	Execute radio self test: No response from radio Set time of day: No response from radio Initiate EFB mode: No response from radio Erase ALL radio messages: Execute DMD self test: Running Rom Test, running Xram test, keyboard test,

The Time Of Day (TOD) is set into the radio , not into the DMD so it looks like this is not a crypto box but rather just a message sending (receiving?) box.

Links

Mike Murphy