Marking a silver coin is likely a mistake, possibly by someone not recognizing the 40% content for a variety of reasons (such as dark rims). But biologists often do mark & recapture studies to estimate populations. Marked critters are released and then recaptured. The ratio of marked:unmarked can be used to calculate estimates of the total population. I just can't imagine that anyone would do such an experiment to get an estimate of the number of silver coins in a total population. Considering that unknown quantities of unmarked silver coins are constantly, but irregularly, added to the total population, some of the assumptions for mark & release studies are not being met anyway.