To get an idea of where the watermark can appear on a stamp, one method is to cut a small rectangular window the size of the stamp out of a small piece of cardboard and place this cutout over one of the printable watermark templates found on this site. As shown in the illustration: the bottom of the letter "S", the top right of the letter "P" and the top left of the letter "S" appear on the stamp. Often only tiny pieces of several letters may appear on an individual stamp, making identification quite difficult.
It is fairly easy to verify that on a normal-sized Washington-Franklin stamp, at most three letters are possible on a single-line watermarked stamp and four letters on the double-line watermarked stamps. The watermark can appear in any of eight orientations.