Hi Rick,
I trust you received my email response yesterday?
It's a Panasonic CR1220 3V or equivalent. We use the equivalent one from Renata ourselves.
The 12 code refers to the fact the battery is 12.5mm in diameter. The 20 refers to it being 2.0mm in height.
Duracell will refer to it as a DL1220
Energizer as a ECR1220
Maxell and Sony both refer to it as a CR1220.
For example, see
https://www.digikey.com/en/products/filter/batteries-non-rechargeable-primary/90?s=N4IgTCBcDaIMICUC0BGMYAMIC6BfIA
See "Appendix F - How to replace the Lithium coin cell battery" of the User Manual pages 233-235
https://www.wildcard-innovations.com.au/documentation.html