Search option should pop up more then a few results
It is a vinyl sticker that fits directly on the switch casing. It should dampen the sound (pinging) of the switch and also stop the dust from entering the switch case. It should work nicely with lubed switches.
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(http://i.imgur.com/lnwhM.jpg)
Btw soulfree914 was kind enough and enlighted me about these:
The sticker is used to eliminate the small movement (left-to-right) in the plastic part of the switch, so it is not something major as oiling & greasing, but the sensitive keyboard enthusiasts could not stand that tiny flaw and made stickers that fit perfectly in that small crack of the switch. lol
My experience was that, after applying the switch, it did fill up that small niche and eliminate the unneeded horizontal tilt from the plastic part of the switch. And I believe your point on preventing dust contamination is another good observation. But sometimes it is too much of a job for such a small, mostly unnoticed effect.