Ron Guth: One of the major varieties of 1806 Half Dollars is the Knob 6, Small Stars. The 6 is easy to differentiate...the top stem ends in either a point or a knob. The stars of interest appear on the obverse, not the reverse; they are either Large or Small. The size difference is readily apparent when the two varieties are placed side-by-side, but the key differences are whether the stars are crowded closely together (Large) or whether the star points are well in from the outer border (Small). Three varieties (O-106, 107, and 108) comprise the Knob 6, Small Stars variety. All three are scarce to rare; collectively, they are scarce compared to the single Knob 6, Large Stars (O-105) variety.
The PCGS CoinFacts Condition Census ranges from AU-50 to AU-58, and includes a single PCGS MS-63 as the finest known.