I believe it's a plug-in from an EMI
WM8 'measuring oscilloscope'.
I can't find a single photo of a WM8 on the web, but here's an earlier
WM4:-
Note the panel meter. This enables more accurate measurements than could be achieved at the time using the screen graticule.
As the amplifier gain is switched, the range of the meter is effectively changed also - hence the switch markings.
The later
WM16 replaces the meter with a calibrated shift dial:-
(From:
https://sites.google.com/site/squirr...st-instruments)
Cheers