Looks like the usual 'T' arrangement. Top row is units, middle is tens and bottom is hundreds. You can see the letters A-G on the units header pins which correspond to the bars on the 7-seg digits:

1  a
2f   b
3  g
4e   c
5  d

Jumper from the center pin to either 1,2 or 3 to make the bar come on in low speed, high speed or both. Remove the jumper (or turn it so it's only on the center pin) if the bar shouldn't come on at all. Looks like it's current set for 8 in low speed and 66 in high speed.

I will still have to do a bit of try and error untill I fully understand how it works but at least I won't fuse the display.
Many TNX, this brings some (Turbo) light into the darkness 😀

A word of advice. This is a single-resistor board. Do NOT test it without having many of its LEDs lit! You will burn them if you start configuring them in isolation.

If you have to try them in isolation, then throw in a resistor on the line to reduce the voltage while you have a few LEDs lighting up. Overshoot the resistance then go down until you get a faint but recognizable brightness. Letting the cell get subjected to higher voltage will cause it to either burn or discolor forever.

Once you have the entire number figured out (e.g. 33, 66), you should be able to remove the additional resistor and get the voltage divided across your chosen cells without fear of over-voltage.

Thank you for the advice.
In that case I will try to figure out the combination first and try it with the fully (pre)set jumpers.