In case this helps anyone else out. I was looking to replace the battery on my Boox Max Lumi but couldn't find any replacement batteries for the 1st gen specifically. Turns out the Max 2 battery replacement fits perfectly in the pre-existing battery recess. The battery connector also fits, although I did have to extend the cables.

The battery part number is "2588158".

I took a picture but I'm unable to upload it.