Old Cambridge vario

I've got an old Cambridge Electric Vario ( 5mts) which model or serial
number can't be found in any part of the instrument. It has a 3
possition switch to change the scale (0.5, 1.0, 2.0).

Behind the instrument there is an hexagonal plug (apparently for the
audio device).

It was working fine until we sent it for callibration, apparently
something worng was done and it doesn't work any more.

As there is no available information about this instrument, I would
very much appreciate if anyone could send me a manual/electric
diagrram/ and also a reference about how to connect the audio device

This instrument is very simmilar to the one shown in the folllowing
link. (third picture and without the box)

http://www.wingsandwheels.com/page20.htm

All help is wellcome

Thank you