100 YEAR OLD BLACKBOARDS

I was fascinated by this story of a school in Oklahoma - Emerson High that was getting new blackboards and they found old ones in behind from 1917 with chalk drawings etc.

The last image intrigues me.  Firstly, the 15 star of davids and A MULTIPLICATION WHEEL.  Someone else on the internet wondered about the wheel as well and here is a link to how they think it works:

https://www.quora.com/How-does-the-multiplication-wheel-that-was-discovered-on-old-chalkboards-at-Emerson-High-School-in-Oklahoma-work?ref=fb