I was browsing some web sites about LLMs and found a link to The Cardboard Computer, which turned out to be about circular slide rules. I have no idea why these are are supposed to be related to LLMs or any other modern AI work, since the first slide rule was invented in 1632. I still have the circular slide rule I bought in high school (1964-68):

15″ refers to the circumference of the inner circle, with the D scale. I also have a few linear slide rules.