A collection of more than 3,000 inverted stamps has sold at auction in New York for more than $5 million.

The two-day auction by Spink Shreves Galleries ended Friday. The collection was amassed by Pittsburgh stockbroker Robert H. Cunliffe, who died last year.

Charles Shreve, president of the Dallas-based gallery, says it was the most comprehensive collection of inverts ever formed.

Inverted stamps result when different colors and elements are printed in separate press runs and a sheet gets flipped upside-down between press runs.

Cunliffe's collection included an "inverted Jenny," a famous U.S. stamp from 1918 featuring an upside-down biplane.