The adapter circuit in this repo carries an inexpensive CPL2102 USB breakout board, cheap and plentiful on ... an onboard MAX3232 chip converts the TTL signals to RS232 via a Cisco-pinout RJ45 ...

Did it have a little adapter in the box? There was a time when if you bought a USB mouse, in the box was ... it may well have come with an adapter for a 9-pin RS232 serial port.