Whether 6V vs. 7.2V matters depends on how they set up the circuit. I would doubt its a huge deal, because batteries don't operate at a constant voltage anyway. The 6V pack was probably more like 7-7.5V when fully charged, and drooped down below 4V when empty. In between it lingered somewhere around 6V for a while, when reasonably loaded.

You could just put together your own set of 5 C or D size NiMH cells and get 6V if you wanted, which would allow you to use a normal off-the-shelf NiMH battery charger in which you could charge AAA's and AA's for other devices as well.