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. 
It bears a family resemblance to the Text Tell PX 1000 (Crypto Museum).

Has QWERTY keyboard, but no backup battery and no "zeroize" keys.  I'm guessing this is for composing messages, and probably receiving messages.
Since it has a TOD (Time Of Day) there may be a way to schedule when a message is sent.

Photos

Click on above image to get a large version

The cylinder at the top holds three AA batteries. The connector mates with an AUDIO/DATA U-229 type on a radio.

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.

PCG-68 Programmable (Portable) Code Generator

Background

2025 October: Found this on the Fair Radio's eBay store (the Ohio store closed in the past year). 
The PRC-68 Family of hand held radios were replacements for the PRC-25, PRC-77 and other FM VHF low band military radios.

The PSG-68 is mentioned on my web pages:
PRC-68, PRC-68A, PRC68B,  PRC-128, TSEC/KYV-2(A).
I don't remember (it was bout 25 years ago) why I named it the Programmable Code Generator.  Note this is a Magnavox device and has no government markings, i.e. the "PSG-68" name was chosen by Magnavox.  Their name does not conform to the Joint Electronics Type Designation System (Wiki) which would be (P)Portable, (C)Carrier, (G )fire control or search light directing.

Photos

Fig 1	Fig 2	Fig 3 Hold up battery cap has spring loaded plunger.

Description

I think this is some sort of message device and probably one that can encrypt and decrypt text messages.  The messages would be sent over the voice channel of a PRC-68 or another radio in that family using the NATO audio connector.  Note this is different from the KYV-2 which makes use of the crypto connector on the bottom of the PRC68 radios.  That means this box also contains a low speed (voice channel) modem.

There is a "zero" button in the upper left of the keyboard and a spring loaded cover to prevent accidental use.  There is also a keep alive battery that's separate from the 9V battery that powers the box.

The keyboard is a 4 row by 9 column (36 keys) where the alphabet is arranged "ABC..) instead of a QWERTY keyboard like the DMD above.  There are function shift keys "NUM" and "LTR" but only the numbers 0, 1, 2, 3 & 4 are provided. 

The display shows all "A" when powered up for less than a second then goes blank.  It's a single line display of maybe 20 characters.

The polarity of the battery is consistent with DS-102 crypto fill devices.  I think this device was made around the same time as the PRC-68 radio (1975).  I don't think any of the PRC-68 family of radios can use a DS-102 key.  So wonder why the negative polarity?  Maybe the PCG-68 will accept a DS-102 key?

Power

Main power is provided by a standard 9V battery.

Memory keep alive requires a cylindrical battery that has a diameter less than 0.459" and a length around 0.573".
The "N" battery has a max diameter of  0.472 but a min dia of 0.429 so might fit, but also may not so probably not it.
The BA5372 (the current NATO keep alive battery) has a diameter of 0.66" max so would not fit.

Note the BA-1372 Mercury battery was replaced by the BA-5372 Lithium battery.
The BA-1372 was made by stacking four (or maybe five) 625 Mercury coin cells, adding a sheet metal dimple contact on both ends and wrapping with paper.
A modern equivalent could be made by stacking four PX625 cells.

The memory battery may based on stacking a smaller coin cell, like the LR44 (0.44" dia).  They drop in, but would need insulation to prevent shorting out. It turns out there is an insulating sleeve so coin cells could be dropped in.  But when powered on no voltage can be read on the center terminal at the bottom, so the polarity of the hold battery is not known.

There may be a way to power the PCG-68 using one of the connector pins.  That's possible on the PRC-68 radio.
When switched "ON" connector pin "A" reads +8.5 volts to the case metal.  Note pin-A is classically the ground pin for audio accessories.  But negative voltages are the common way crypto keys are loaded.

Related

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
------------------------------
Crypto - General Cryptography
CryptoM - Cryptographic machines
FS5000 Stay Behind Spy Radio

Links

Mike Murphy (Ebay)
Fair Radio's eBay store