I recently liberated a Passport MIDI Transport. It is about 1 inch high and seems to be designed to allow you to use both your modem and printer serial ports for MIDI. On the front are a power switch, a Thru button plus In and Out LEDs for the printer port, same thing for the modem port, a Sync button (FSK or MTC), a Mode button (Read or Write), and a Lock LED.
On the rear it as a standard power connector, 4 Mac serial ports (printer port computer and thru, modem port computer and thru), 6 midi ports (modem port in and 3 outs, printer port in and out), and Tape in and out which look like 1/4 inch phone jacks.
Google didn't turn up much on the web. I'm interested in finding out more about it.
http://katsmacs.blogspot.com/