The 1892-O Micro O half dollar is the rarest in the Barber half dollar series. It is believed that the mintmark punch used by the engraver was meant for a quarter dollar rather than the half dollar. A population study was undertaken about 2003 by a member of the Barber Collectors Society (Michael Flanagan) and he published all known examples (certified and raw) for a total of just over 50 coins.
The 1892-O Micro O half dollar is usually well struck but often comes with a bit of weakness in the upper right shield and on the eagle's wing. Uncirculated examples often have proof-like or semi proof-like fields. A die crack through the date is seen in most examples. It is believed that the improperly-sized mintmark was discovered fairly early and that the die was then taken out of service, thus accounting for the relatively few examples that are known. This variety was discovered not very long afterwards and examples were sought even in the early 1900s.
-- Contributed by Dr. Peter Shireman