The contamination shield was just rotted and seems to have actually captured and held moisture under it.
The underside is completely rusted as well, the rubber shock guard between the switches and the plate is disgusting and disintegrates when handled. Switches are surprisingly OK, but a bit scratchy from particulate that got in there.
Some ID on the back of the sense plate assembly. German board, made in Amsterdam ?
After sanding the plate it still shows signs of discolouration. I wanted to go to a raw steel look, but decided I would paint it because I just couldn't get rid of the color difference with the tools I have.
Using a steel punch to make a new rubber mat for the back of the plate. Tiresome but necessary because otherwise the switches were too loose.
Painted plate and finished rubber mat.
Disassembled all the switches to clean them. The metal stems and springs were given a vinegar bath to clean off the rust. The plastic housing just cleaned out with soapy water and the plastic stems gently wiped down with antistatic cleaner.
Before and after example of a switch
Cleaned switches in the painted plate. Things are beginning to take shape.
Keycaps were cleaned with just some luke warm soapy water and fitted. With the addition of an Xwhatsit controller it is now actually useable !
Befehl ist Befehl !
Reassembled it can now be used, but it's still ugly with it's busted case
Planck for scale 
I decided to use plastidip to "paint" the case. This worked reasonably well. It's easy to apply and could be used on both the plastic top and metal bottom half of the shell. Just see the color difference. It's not snow white either. The dip is white in natural light and strangely slightly beige-tinted in artificial light. Perfect for this keyboard.
Top and bottom done.
Full view of the dipped top. It actually filled in the ugly yellow splotch nicely. You can hardly even see it anymore except when it catches the light if you look very closely at a certain angle. The white also makes the discolouration on a couple of the dark grey keys look less noticable, that's a bonus.
The finished keyboard still has some issues: the controller connector doesn't seem to grab onto the sense plate properly causing it to type garbage sometimes, solved by pushing on the controller. I'm thinking of maybe layering some solder on the sense plate connections to make them thicker.
This beast is so big I don't have a good place to put it though, it ends up being a bit too high up for comfort on my desk.
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