Are you planning to take 7.2 baterries and use a register in series to drop down the voltage whatever your flash calls for?

I have a 45 something (I'm at work) and as I recall, it takes 6 batteries, which makes it 9 volts. It also has a different battery holder for rechargables.

Please know that using registers for this purpose works very poorly. The voltage drop across the register will be current dependent, so when the flash is charging at full, the voltage will drop more and when it's fully charged, voltage drop will be zero - meaning full battery voltage will be applied to the flash unit.

Also, whatever the voltage register drops will be dissipated as heat. It can heat up significantly.