IIRC that's like the old gladius... and will only work properly on a pair of 3V cells. Which creates an interesting obstacle. You wind up having to choose whether to use a pair of LiFePO4 cells, or a pair of voltage regulated protected LiCo cells, both with a long list of disadvantages. (someone please correct me if I'm wrong on this)
I tend to just recommending trying to find a suitable replacement that runs on 18650 3.7V cells instead so you can actually get comparable runtime in a rechargeable format of similar size.