The blue octal-canister type plug in that was somewhat common in broadcast and other audio production environments in the 1980s. 

Specs for this model are 150 ohms input xfmr balanced; 600 ohm line out unbalanced. Gain can be adjusted 25-50 dB with resistor value choice (R-ext). The guide key is broken on the octal-type base but you can eyeball it - pins and electrical connections are still good. 

Thank you for looking!