More than a decade ago, Diomidis Spinellis created an implementation running on the OLPC, written in Squeak:

https://www.spinellis.gr/sw/ameso/

Backwards engineering the front and back and how it all came together and passed though has me picturing this as a possibly the earliest front end/back end development situation. It even included documentation. A software version would be neat to see.

I feel like I saw one on the F-Droid app repository

I was thinking the same thing! I 3D model would be really neat to watch and pick apart.

I have one on mu iPad. There are 2 in the App Store, I think.