Re: Foundation Passport (FE) hardware wallet review and walkthrough
It seems to me like the Li-Ion battery in the keystone is basically 3 rechargeable 's stuck together in a piece of plastic. That would have been possible e.g. as a 'replacement back piece' for FE.
In Keystone Pro you get one slim rechargable Li-ion batter yon the left so I am sure it's impossible to get any battery cramped inside.On the right side you are getting fat back part replacement battery holder that can use AAA batteries, and this makes your hardware wallet have much bigger back.
Here is one review by John Chow dot Com from 2021 so you can see the BIG difference:
https://www.youtube.com/watch?v=mAV0kchQF-g
