The first "filigree" (pierced work) overlays were introduced around 1898, and were made of silver. Waterman was probably the first to offer such pens, and their early models are typically marked as fine silver that is, 99.9% pure, rather than the 92.5% of sterling. The main reason for this appears to be that the silver was put on the pens by electrodeposition (similar to electroplating, but leaving a much thicker deposit).
The advantages of plating on an overlay are obvious, but there were disadvantages as well: fine silver is soft, and prone to wear and damage; the overlay is necessarily thin, and the electrodeposited metal tends to be grainy. For these reasons, electrodeposited fine silver was eventually displaced by overlays cut and pieced together from sterling silver sheet (Waterman seems to have made the shift around 1907).
The differences between the two kinds of overlay are readily apparent to the educated eye. Interestingly enough, Waterman was just about the only maker to mark its electrodeposited overlays as fine silver; the vast majority of such pens are normally found marked "Sterling" which standard of purity they meet, of course, with room to spare.
For more on electrodeposition of overlays, see the 1909 article reprinted in The PENnant VIII/2 (Feb 1994), as well as the commentary in the following issue; also see Fingerman, "Early Silver Filigree Waterman Pens", PENnant VII/3 (Mar 1993).
NOTE: the question of how overlays were made is far from fully answered. Some Waterman fine silver overlays, for example, have clearly been made from sheet, and have had their piercings cut with a knife while on the pen. In these cases, it seems that the softness of fine silver was what recommended its use.