JW wrote:

Just a WAG, but perhaps in normal operation the dim digits were constantly
lit, while the bright ones weren't?

could be, but I doubt it. The brightness starts going down from digit 10 and
it is really progressive, each digit is dimmer than the one on its left.
Things without a schematic are difficult sometimes.
What really puzzled me is the fact that swapping the filament pins didn't
make any difference. I would suspect the driver (Rockwell 10937), but
grid waveforms look all the same.
Frank