This is A mysterious Tektron secure telephone

Recently, a mysterious telephone was offered for sale at eBay. The device was made by the little-known company Tektron Micro Electronics, Inc. from Hanover, Maryland, and seems to be a secure phone for military use.

Apart from the pictures shown below, nothing more is known about it, but maybe some readers of this weblog recognize the device and have some more information about its purpose and where it was used.

A Tektron secure military telephone
(Photo via eBay - Click to enlarge)

The phone comes without a handset, but it has a display and a common 12-button key pad, with some additional special purpose buttons. According to the seller, all of them are made of some kind of rubbery material instead of hard plastic. The big round buttons reveal that this is a secure phone, capable encrypting the calls: a green button with a green light for Secure and a red button with a (probably) red light for Non-Secure:

Keypad of the Tektron telephone
(Photo via eBay - Click to enlarge)

It seems the small button with "2nd" can be used to select the functions which are marked in blue above the standard buttons. Most interesting are the FO (Flash Override) designation above the "3", the F (Flash) above the "6", the I (Immediate) above the "9" and the P (Priority) above the "#" button.

FO, F, I, and P designate the four levels of a system called Multilevel Precedence and Preemption (MLPP), which allows to make phone calls that get precedence over ones with a lower priority. Flash Override (FO) was designed to allow the US President and the National Command Authority to preempt any other traffic in the network in case of a national military emergency.

This precedence system only works on telephone networks that allow this special capability, like the AUTOVON network that was used by the US military (since 1982 replaced by the Defence Switched Network). One of the characteristics of the AUTOVON network was that most of its phones were equipped with a standardized keypad with four extra red buttons for the precedence levels:

The standard AUTOVON keypad
(Click to enlarge)

So apparently, the Tektron phone was intended for use on the military telephone network, but why it doesn't have the standard AUTOVON keypad is a mystery.

We also don't know when the phone was manufactured. The only indication is provided by the label on the back of the device. It says the model number is EXT-4Rx and has the serial number 271/4.0. The seller had a second device with serial number 111.

There is also a National or NATO Stock Number (NSN): 5810-01-357-8193. Looking up this number on a stock number website returns a "Date Established" of 1992. This indicates the phone must be somewhere from the 1990s, although the way this number is placed, without its own line, also looks like it could have been added later on:

Label of the Tektron telephone
(Photo via eBay - Click to enlarge)

It's not known where exactly this phone was used, which is an even bigger question because in the 1990s secure telephony for the US government and military had largely been standardized after the introduction of the STU-III family of secure voice products.

The STU-III standard was introduced by the NSA in 1987, and three manufacturers were allowed to produce secure telephones based on this standard:

- Motorola
- AT&T (later: Lucent Technologies > General Dynamics)
- RCA (later: General Electric > Lockheed Martin > L3-Communications)

Motorola and AT&T each made a few hundred thousand of these devices. Tektron is not known for having participated in the STU-III program.

Side view of the Tektron secure military telephone
(Photo via eBay - Click to enlarge)

The Tektron secure phone measures 7.75 inches (19,6 cm) wide, a little over 9 inches tall (22,8 cm) and 2 inches (5 cm) thick. The encryption function made it very heavy: it weighs about 5,5 pounds (2,5 kg), as the case is fully made from cast non-metallic metal, perhaps aluminum.

Such a metal encasing prevents electromagnetic radiation from being intercepted from the outside (TEMPEST). The STU-III, and the newer STE phones only have their bottom part out of metal, with the upper part out of plastic.