The next batch of BBs will have the slightly longer bottom section, so that 2 x protected CR2 3v batteries will be able to be housed in the chamber.
But stacking batteries in series is a practice that we can not endorse or recommend, as it can increase the chance of battery failure. The BB was designed to take one 14500 3.7v protected battery only. But people were using non-protected CR2s, regardless of the advice given. So it was decided to make the bottom slightly longer so that the chamber can accommodate protected CR2s, if the user decides to use the device in this configuration.
A limited quantity of the new longer BB bottom section will also be available separately.