4 layers35.1 × 76.3 mm

Tiny4FSK
New-England-Weather-Balloon-Society/Tiny4FSK
Horus Binary v2 HAB Tracker
90 parts96CERN-OHL-W-2.0
The BK-53 is a button And Strip Battery Connector made by MPD with 2 pins. - Button And Strip Battery Connector ROHS.
JLCPCB lists it (part C5597032). 2 open-source boards in 2 repositories on GitHub use it.
The library has no symbol for this part yet.
The library has no footprint for this part yet.

New-England-Weather-Balloon-Society/Tiny4FSK
Horus Binary v2 HAB Tracker

MPD publishes it: the Datasheet link on this page opens it.
JLCPCB lists it as part C5597032, with 0 in stock; the sellers' links on this page open their listings.
2 boards in 2 repositories on GitHub; the most starred are listed on this page, with their KiCad files.