My guess is also PATA, seeing that notch in the middle and 40 pin count.
It was probably for a optical CD drive which used PATA seeing how its a additional serial port for Legacy peripherals, Ie.. A Optical CD drive or possibly a Tape Drive.
I am guessing it had its own external enclosure which had a independant power supply or brick, and you just connected the two devices via the PATA cable.