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There is a current source circuit on the CPU card that can affect the display level. It is composed of a matched pair of transistors.
Also the ROM0 chip that drives the display anodes is powered through a resistor. The value of this resistor (or the chip itself) also affects the display level.
Also the cathode driver chip on the keyboard can affect the display level.
Finally the middle horizontal segment of the displays is driven differently than the other segments. A difference in the brightness of this segment vs the others may may be adjusted by varying one of the resistors on the keyboard circuit.