Pot wipers are often made of NiChrome, or Nickel chromium. This is a very valuable resource that is fast diminishing, and manufacturers often take to recycling other products that they make, in order to waste nothing.
Therefore, they take all the money they make from manufacturing carbon film resistors, and convert it to US currency, five cent pieces, specifically. These are then stamped out into corresponding shapes, such as wipers for pots and nichrome wire for wirewound resistors. Those numbers you see are simply markings from the previous form of nickel. Maybe the date on the coin had a 3 or a 9 or something. I have a pot that has Thomas Jefferson's nose on it.
(also, nice high-res shot, is that pot medical?):na: