The Psion Organiser was a range of British designed and built Personal Digital Assistants.
This is my Psion Organiser 2 LZ. After 33 years it is fully working and is in semi-retirement as an alarm clock - still waking me up every work day morning.
The lower case slides down to reveal a alphabetic keyboard.
There are two Datapak slots on the back. This one has the Pocket Spreadsheet
For scale, the Psion with a Raspberry Pi Zero 2 in an official case.
More technical descriptions to follow.
I also own a second hand (retired) LZ 64, however it makes a buzzing noise when switched on.