Vintage computer repair is often a tedious task. Currently I am putting all the keyboard keys back on this Psion after reconnecting a loose cable. The keys are just sitting on top of this rubber membrane and then a plastic top cover holds them on.
@ajroach42 @lroop I had one (still have somewhere...) - actually the Acorn branded version - but I used it heavily in ~95; mostly for taking notes in the university library as I worked on my PhD; thumb typing on them was just great; I'd still recommend that keyboard over modern touchscreens. I stopped using it after a few years when the hinge broke which was their normal weak point.
IIRC that's actually how IBM/Lexmark/Unicomp assemble buckling spring keyboards - it's all put onto the top plate upside down, with the heat stakes acting as locating pegs for the membrane.
@lroop I had similar issues when I opened up my Sharp PC-1261 http://atomized.org/blog/2018/12/02/repairing-a-sharp-pc-1261-pocket-computer/