Some time ago, one of my colleagues very kindly brought in his Psion MC 400 to let me try it out and take some photos.
Considering it was released in 1989, the device is well featured, even sporting a trackpad (though the absolute positioning proves somewhat confusing when you're used to today's laptops).
Another feature that made it across to the Series 3 devices was the Psion key though, disappointingly, it was replaced with the far more generic 'Fn' key on the Series 5.
Just like the Psion key, many charming design elements of the Series 3 have their origins in the MC series of devices. The badging, colour accents, typography, and ridges in the plastic housing (seen above), can all be found in the Series 3.
The MC 400 even sported Psion's own 'SSD' external media, which could be found in both a write-once flash version, and the a expensive read-write battery-backed form.