DES modem

[SINCLAIR DOUGLAS N]

I recently aquired a DES modem.  It's a MESA432 made by Western Datacom.
The writing on the front proclaims "9600 bps NBS DES ENCRYPTION MODEM".
It has all of the usual front lights, plus one labled 'encrypt'.  On
the back it has jacks for leased-line and dial-up, an RS232 connector
and an external speaker connector.

Has anyone ever used one of these?  Can anyone give me any information on it?
I realize that without a second modem it's not much use.