Unlike other dates, no 1866 circulation strikes were made for the half dollar, making proofs the only 1866 Seated Liberty mintage.   

During 1866, the "In God We Trust" motto was added to the reverse, making the no motto 1866 half unique in the series. 

Proofs were exclusive collector sets until the late 1800s, giving the 1866 Seated proof a high perceived rarity.

Records indicate roughly 50 proof examples were struck of the rare date 1866 Liberty Seated 50 cent coins.  

Today, experts estimate only about 10-12 authenticated proof examples survive of this one-year type.

When available, the 1866 proof historically sells at auction for over $100,000 even in impaired proof grades.

Many theories attempt to explain the unusual origin of this proof-only half dollar issue.