A buddy of mine has both serials up and running. I haven't got to make the mod yet.
Here is his first pass on the pin-outs. They might not be 100% accurate.
Looking at the mobo such that the CPU is on the top right:
DCD w/pu RX w/pu
CTS RI w/pu
? audio in
CTS ? DSR TX DCD +5
RI - RTS DCD RXpu GND
Obviously the modem connector is the vertical one, the serial header the other. There is likely an LED hookup on one or both of the ? pins on the modem connector.