Hopefully Jack Haddock will respond to this thread. It was because of him I mentioned the Tiger. At the time I bought my no mint tiger from him, it sounded like all the original tigers had the mint mark. Then, apparently, the die got plugged up by a "floater". Hence, all the early no mints had small craters in them in place of the mint mark, but a "no mint" nonetheless. Little by little the excess apparently wore away until it became flush with the surrounding die area, with the subsequent strikes showing no mint mark nor crater at all.
Where are you, Jack?