I'll answer quickly myself - figured out something by experimentation:

PIN Function
10 GND
28 SOUT1
29 /RTS1
30 /DTR1
32 /RSLD1
33 SIN1
34 /CTS1
35 /DSR1
36 /RI1
38 GND
42 /COM2 PD
46 COM2 PWR
47 +5v
49 +5v
50 GND

Above pins 28 - 36 are wired straight to the WD76C30 IO chip (above the SIMM slot #1) and are serial port COM2 at TTL level.
PIn 42 is a presence detect pin which when pulled low indicates that the serial adapter is installed. This gives as 162 message at bootup and adds the serial adapter items to the setup menu.
Pin 46 is a power control pin which is tied to the menu item that turns the serial adapter on and off in the setup menu and in the PS2 program.

Didn't figure out the other pins, and didn't try since I'm mostly interested in the 2nd serial port. Hooked up an arduino for testing:

What I'm looking to do with this is to add a serial MIDI interface and a wavetable board internally 😀

Greetings,
Timo