Keyboard three reassembled
I've learned a bit more - don't bother with replacing the rivets at the front edge. Also - note which places on the keyholder, don't have a buckling spring assembly; mark with a red cross, otherwise you won't know which ones to leave empty when you reassemble.
So, I reassembled the keyboard and tested it. It worked ... except for the letter U. Clearly, I can't use a keyboard that is missing the U, although I can create a U using the alt key and the numeric keypad, but that's clumsy.
I think the problem would be that during the reassembly, I didn't get that bucking spring assembly right. So, I'll have to dismantle the keyboard and try again
The original problem that this keyboard had, is solved. I think it was the mess on the plastic sheets
It'll be much easier second time around - no chiselling or drilling required, and no washing. Also, I've been gradually collecting tools to make the job easier. Plus, practice makes things easier.
I'll use "showkey -k" to test the keys. Also the grey + key on my main workstation keyboard wasn't working. I took off the keycap, and prodded the bucking spring assembly with a plastic prodder, replaced the keycap, and now it's working!