With a bit of fiddling but, most important, thanks to the invaluable help of the remains of Gralex Industries, I could light up the (most probably custom made) Gralex Panel Meter.
I had to provide AC, but I did not want to run it right off the 220V mains. Also the panel supports four voltages, as low as 100Vac (then 117V, 220V, 237V), so I connected back-to-back two transformers until I got somewhere close to 117V.
The instrument is a millivolt meter in all respects, with a range up to 3999 mV. The positive range seems to be defective since it stopts at half scale 1500 mV.
Can it work as a clock, since the original case is now gone? It could and I see two possible conversions: either feed something into the digital circuitry or provide a suitable (negative) voltage.
The first option is hard without a schematic diagram. The second option needs a DAC with a pretty good resolution.
Time will tell.