There were also some early shoe-based devices I have read about, which used earpieces (difficult to avoid breaking the thin wires necessary to hide them, and prevent damage from sweat). Some of these stories unfortunately weren't documented super well -- I think I came across them from the original participants chatting on a long-defunct forum or newsgroup -- but it is mentioned in passing here: https://jimsudmeierstories.com/adventures-with-a-concealed-b...

> Then around 1976 came “David,” using the Z80 microprocessor, oriented towards team play (the Big Player making the big bets) with hand keyboards operated through holes in pockets and transmitters to signal the Big Player. Later came “Thor,” a computer to track the shuffling (and possible clumping) of multiple decks. One of his inventions involved networking players together with fine wires about 3 feet long. Then there were “Magic Shoes” in which 12 batteries, computer, and all were hidden in “Frankenstein” shoes. Later still there was “Narnia, the sequencing computer.”

The inventor Keith Taft talks about it in more detail in an interview here: https://www.lasvegasadvisor.com/gambling-with-an-edge/interv...