i think lead came from latin plumb (or something) which is why we call it plumbing cuz it was made out of lead (until we found it, it makes you crazy) which led to the romans going kinda batty, lead is bad, do not want.
i mean it protects you from radiation, while that's good it's also trouble, even Superman can't see through lead.
Pb stands for "Plumbum". In highschool, I went and looked up the meanings of all the chemical symbols that weren't immediately obvious.
Pb was pretty commonly found in Leaded glass (which contains a lot of lead oxide) to protect you from x-rays. Now they use other elements, because it's much cheaper and less illegal to throw them out, then.
I don't think you'd absorb enough lead from keycaps to be an issue, especially since lead is probably a small percentage of the total composition (if present at all) but if you're really that worried, get someone chinese to ask the seller.
Or buy one, and do spectroscopy to it. (AES, EDS, XRD, SIMS, Mass Spec, etc.)