I have a few cameras that need a light meter. I have an old Lunasix 3 which was excellent when it was new and mercury batteries were still available.
Of course I would prefer a Gossen Digiflash but it is beyond my budget.
You will find tons of writings about replacing mercury batteries with other types and people try to sell you funny adapters to make alkaline buttons work in the Lunasix and others.
Since my Loonie had a severe corrosion problem I decided to open it and see how it works.
Above is the circuit diagram.
AFAIK the Luna Pro is the same thing.
There are no electronics in the Lunasix 3.
It is just a battery, a CdS cell, a meter movement and a bunch of resistors.
The only nonlinear element is the CdS cell. The rest is linear.
Therefore it doesn't matter if the battery voltage is 2 x 1.5 V (alkaline) or 2 x 1.35 V (mercury) as long as you calibrate the meter with the used battery type.
Load a new set of PX625G or similar.
Adjust the battery check so the needle points to the upper end of the red field when you move the bat.-check slider.
Calibrate both measuring ranges with a known good light meter.
Check the batteries regularly.
Replace when the needle is below the red field.
Alkalines don't discharge at constant voltage like mercuries used to do, so you have to change the batteries more often but alkalines are dirt cheap - they still last about one year of regular use.