1819 Half Dollars are found in three major varieties: Normal Date, 1819/8 Overdate with a Large 9, and 1819/8 Overdate with a Small 9. The Large 9 Overdate is comprised of several die varieties (Overton 102, 103, 104, 105, and 106), but the Small 9 Overdate is comrpised of only one (O-101). The Small 9 Overdate is not particularly rare, but it is definitely the least common of the three major varieties. The Small 9 Overdate offers the crispest view of the underlying 8, even on lower grade specimens.
Approximately three dozen Mint State 1819 Small 9 Overdates are known, mostly in grades ranging from MS62 to MS64. Gems are truly rare and only one example is known in MS66.