Sony ereader lighted cover question

I don't know whether this is the right place for this question.

I have an incredibly ancient Sony ereader - I'm thinking a T2, but my long-ago me put a skin on it, and I can't find the details. It's in a cover with a light in the cover.

I just got it out, and the ereader is charging, so it still works, but of course the light in the cover is dead, and I can't for the life of me figure out how to open the battery case to change the battery. Because this is all so ancient, Google is no help. The slider position at the base of the battery case doesn't seem to do anything.

Can anyone help me on this? Thanks

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