The public is cautioned to research prices before buying advertised state quarter year or proof sets.

The library is home to one of only five surviving proof sets, all of which differ in their composition.

Only five sets were created of the series, together with two artist's proof sets.

In proof sets issued since 1996, the five cent coin is made of sterling silver.

Occasionally, there are errors which escape the Mint's inspection process, resulting in some very rare and expensive proof sets.

Within one week of the discovery, proof sets lacking the mint mark on the cent are being bought for $600.

This year the mint has sold more than 1.8 million proof sets, although only about 3,500 sets are thought to have the missing mark.

At the time, it was announced that proof sets had already sold out when coins went off sale.

Due to the coin shortage, the Treasury Department announced that no proof sets would be struck in 1965.

The Mint continued to make proof sets of trade dollars until 1883.