Raised lead, theses letters stand proud on the surface of the stone, this method is most commonly used on granite and very hard stones.
They are not stuck on. Firstly holes are drilled into the stone behind the letters, the lead is then beaten on to the stone and as it is soft, a proportion of it is pushed into the drill holes, this in turn acts as an anchor to hold the letters in place, the remaining surface lead is then trimmed to shape as required.
Drill and lead, the method is similar to above, save that the letter is cut into the stone first, then the letters are drilled into, the lead is beaten into the letters, the excess is removed to leave a flush finish to the stones surface.
Very commonly used on marble and limestone of a consistent structure, softer stones are usually composite filled (cement) to alleviate damage that would be caused by beating the lead into it. This method is not suitable for granite.