Hmm, this is the tester 165A I'm familiar with perhaps that is a later version. It is a continuity tester (buzzer) and would have been used to prove accuracy of cabling from distribution frame to equipment. Generally, one person would be at the frame end and another at the equipment end, if both within audible distance of each other only one buzzer would be used, if distant one each. One lead from the buzzer would be attached to the first wire of the cable to be tested and the other lead momentarily touched on each wire in turn moving on after each buzz. If only one buzzer used, the other end of the cable would just have a piece of wire connected to the first wire and the loose end touched on subsequent wires. Thus not only proving the continuity of each wire but also correct connectivity. I think I still have one somewhere.
[Moderator edited to trim the white space from the attached image.]