The Brent secure telephone system comes in several varieties.
 
The basic model was colloquially known as an 'X21 Brent' . It  had no direct dial interface, but was dialed via an external Terminal Adaptor that basically put the X21 on-line to the other Brent.
 
The second  version was an 'ISDN Brent' which plugged into a standard UK ISDN line, thus giving it capabilities of clear voice, secure voice and secure data.
 
The third was a Brent 2, which was a ISDN Brent, with a Red S0 on the rear which gave the Brent 2 a 'dial though' capability for ISDN products (so one can do secure ISDN video conferencing for example) via the Red S0.
 
The Brent 2 also came in two sub variants called 'Lector and Hannibal' . It was manufactured by Marconi Space and Defense.
 
The Secure Telephone Key (STK) was the 'fill' and it was inserted in the right hand side. One had to turn it a quarter of a turn clockwise to engage it. The STK was basically an EEPROM.
 
The STK's came in basically 8 variants. Number  0 was Voice. Anything greater than that was classified as a 'Super STK' which also came with a pin code book. One had to type in a specific PIN to get the key to function.
 
Also, the STK also identified the user so that the remote party had an idea who was calling. Therefore when receiving a secure telephone call, on the LCD display, you would see the name of the remote party  - e.g 'HMS' Unpronounceable.

A Brent 2 manufacturer's data brochure can be found here.
 

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Contributors and References:

March 1/10