I believe that these were used (at least for one use) on SAC 'Looking Glass' KC-135 missions. It's basically a teletype but much more weight and volume efficient as airborne service prefer. I was in the USAF in the late sixties performing teletype/crypto maintenance but this unit was not part of tech school so no first hand knowledge other then hearsay. Your unit looks newer then what was flying in the 66-70 time frame because of the ICs.