I recently have found an interface for the Psion Series 3a (it’s declared as designed for 3a and I don’t know if it supports 3c or 3mc). It’s a nice little box that allow to connect the PS/2 keyboard and a printer.
I saw only the keyboard working (there was no printer around to try the printer interface).
The thing can be operated with a normal Series 3 power adapter but it can also run on a 9V battery. So it is portable. It’s a funny stuff.
There can be found also other goodies: a floppy and a parallel printer adapter cable. Unfortunately I’m not aware about these thinks in working state.