Restoring old SR-50 Calculator, odd Charger Voltage
I have that calculator, and the charger.  Circuit wise, there is a diode which ends up in series, and DC is presented to the battery and the calculator.  9V open and 6V +/- sounds right.  I have rebuilt the packs putting in AA nicads.  It helps to fit things in, if you can find slightly undersized cells.  Hot melt glue can be your friend.