(crossposted to DT, as saw it there first)
At first I thought I agreed they were RLC filters to deal with a noisy environment, but notice the last strobe line is heavily filtered - it's the one that is on the left-hand side of the keyboard, and fairly exposed - but it is a strobe, so it's pretty much noise-immune.
The way the IBM F works, it is the height of the voltage reflected through the cap that is measured, not the height of the spike at the front caused by the derivative of the voltage as is used to sense in the xwhatsit controller, or the topre sense - both of which rely on a steep strobe pulse.
topre, xwhatsit sense the height of the spike at the '+' . (this is a rather brutal simplification of what topre does - apologies)
STROBE SENSE
+
_____ A
| | -----> | \___
_____| |_____ _____| \_
IBM senses on the flatter area after any initial spike -
_____ A +
| | -----> | \___
_____| |_____ _____| \_
which will also accept no significant initial spike:
_____ +
/ \ _____
/ \ ------> / \
____/ \_____ _____/ \_____
So - I think that particular messy filter rounds the front and back of the strobe, so it will *cause* less interference with nearby hardware. I don't think it's a device to prevent people from snooping, as the result line back from the sense comparator - a relatively loud digital line - is not filtered.
Addendum:
That strobe line, unlike the others, runs along under the 10 F-keys, right by the edge of the PCB where the grounded steel back and barrel plates won't help as much. Other filtered lines are also powered digital lines, or the actual power - so, not particularly susceptible to noise, though they certainly generate it. Still think it's about ameliorating noise emission, for use near some rating of sensitive equipment, rather than for noise acceptance or stealth. I also suspect the 'EMS' boards are also for noise amelioration, rather than stealth - as there were several medical applications where a keyboard was needed, but digital signals were verboten - EKG rooms used to be Faraday cage shielded in hospitals. (though last time I was in one was nigh ten years ago...) But, if the manufacturers of the EMS boards were also mil contractors, I'd be less skeptical, I suppose.
dfj's $0.05
