This is from a YouTube video that I was watching:
(https://files.granzeier.com/Downloads/SavageCircuits/E404.png)
This is a notice written in the silkscreen layer of the PCB directly underneath the battery case. When you pull the battery out, this becomes visible through the clear plastic of the case.
The video is pretty cool, and he does a great job of describing his handheld computer.
Speaking of 404 errors, did you know that Savage///Circuits has a custom 404 page (https://savagecircuits.com/training/)?