I found out the pinout for the PSION MC600 floppy connector by measuring the trace-connections from connector to the floppy controller IC on the MC600 mainboard.

All control signals from MC600 to the floppy need to be inverted to work with standard PC floppy drives.
All signals coming from the drive except maybe /DSKCHG need to be pulled up to +5V by 10k Resistors.
The floppy drive must be powered from an external power supply, the 5V output of the MC600 connector seems to be too weak and the computer will shut down sometimes, when the drive is getting activated.

I built a test adapter on a raster pcb with 74LS04 inverters instead of the 74LS240.
The prototype works fine and the MC600 boots from a 1.44MB MS-DOS bootdisk.
(you have to keep ALT pressed on reset to get to the boot options menu. There you can choose booting from A:)

The mating connector can be bought from DigiKey:
https://www.digikey.de/en/products/detail/te-connectivity-amp-connectors/5749111-1/807188?s=N4Ig