Very nice! Usually FLS3 are mounted sideways, not upside-down; maybe that's a thing for the clicky version.
FLS are fairly dirtproof, but are not immune to use. Do the caps show sign of having done heavy duty?
The caps look pretty good. The board was "restored" so it's possible they were replaced, but I don't think it's likely.
Pulled out my macro lens and got some shots of the keycaps.
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(https://i.imgur.com/Fn0QOad.jpg)
I'm not that good at telling one type of printing from another, so feel free to voice any opinions y'all might have.
I don't see any yellowing or shine on these, so my instincts say PBT.
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(https://i.imgur.com/s1ghQUK.jpg)
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(https://i.imgur.com/sGpRhLK.jpg)
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(https://i.imgur.com/RWqqe4X.jpg)
The stabilizer mounting method is a bit novel. This slider clip things are held in place by the keycap mounting to the slider. I bet they're a bugger to reassemble.
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(https://i.imgur.com/5lJKnbA.jpg)
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(https://i.imgur.com/3hNL789.jpg)
With it down to bare board, I've been trying to trace the matrix, but I'm a bit thrown. My multimeter's continuity mode won't pass through the diodes in either direction. This is probably obvious to anybody who's more electrically minded than I. Gonna do a bit of research and think.
Diodes can be tough to check when installed in a circuit. Its possible that you are passing through the diode in the fwd bias direction, but going back through a different path around the diode in the reverse bias direction. That or the diode is shot, but id be hesitant to think that.
Edit: Doh, you said it wont pass through.... ignore my post.