This is an insert for the original Texas Instruments SR-56 calculator battery compartment. It allows you to take out the old (and probably leaking/non-functioning) rechargeable batteries and insert your own replacement lithium battery and charger.
I started this project while looking for ways to repurpose batteries out of the disposable vapes that are popular right now. I'd been harvesting the batteries for a while, and an old SR-56 was the first thing I experimented with.
The battery is just a generic lithium battery about the size of a AA, I can't link it specifically because I harvested it from an old vape. The charge controller I used is this one from Tayda electronics I happened to have on hand: https://www.taydaelectronics.com/57170-dup-tp4056-lithium-battery-charge-control-module-dc-5v-1a-with-protection.html
It looks like at one point the original battery pack had pieces of plastic spanning across its length, however I received mine with these already broken off. If yours are already broken, or you don't mind breaking yours off on purpose, then this case simply sets right in to where the old batteries went. I secured mine with a piece of double sided tape and that was it.
I did decide to drill holes and screw my charge controller down instead of gluing or taping that. I was able to re-use a couple screws that came out of the same vape I disassembled to get the battery.