A PCB silkscreen reading T21 Internal Tag, Fixed Battery Switchable, Part No. R2916-219 PCB Rev V2
https://digipres.club/system/media_attachments/files/115/488/965/140/810/976/original/c5aca206e7c2256d.jpg
It's officially called a T21 Internal Tag Fixed Battery Switchable
(some models don't have the switch)
part number R2916-219
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