There is this RS232 Relay with 'www.iorelay.com' printed on the PCB.

I believe the manual for it is this: https://s3.amazonaws.com/assets.controlanything.com/manuals/R2X.PDF

On Linux, to turn the relay off and on via serial:

## Ensure serial is at 9600 baud
# stty -F /dev/ttyS0 9600

## Toggle it
# while true ; do echo -e "\xFE\x01" > /dev/ttyS0 ; sleep 1 ; echo -e "\xFE\x00" > /dev/ttyS0 ; sleep 1 ; done

## You can also use printf
# printf \\$(printf '%03o' 254) > /dev/ttyS0
# printf \\$(printf '%03o' 0) > /dev/ttyS0