My Elecraft KX3 radio's computer link cable is almost the same RS-232 over TRS pinout that the ET312 uses. I think one pin is different, though, and had to build a sort of crossover cable.

Ok, since you're familiar with the ET-312...

You know we reversed the ET-312 to the circuit level, right? :)

https://github.com/buttshock/mk312-bt

Yeah I've looked at that a number of times. I always thought it was cute that they went to so much effort to scrape the part numbers off the ICs.