lead would only be in the solder of the wire, and on the small circuit board, lead does not make a good switch, it oxidizes to easily, keyboards need to be good switchs, and since the current of these switchs is very low, they use materials in the switch for low current, and reliability, for some equiptment run by a computer with keyboards reliability is very inportant in that case you can find some of the more valuable metals, the silver on mylar is usually a dull gray, black can be carbon, I have not seen any silver sulfide in them, some of the key boards you find can have switch's with gold or palladium spring disks, some of these have many switch's soldered (with lead to a circuit board a propane torch to remove) open the switch for goodies.