This variety is an overdate that shows traces of an 854 beneath the final 1855 date. The arrows at each side of the date were placed there to signify a reduction of the weight of the coins in 1853. The arrows served as a quick identifier so that the old, higher weight coins could be removed from circulation quickly.
The overdate is roughly two to three times as rare as the "normal" date. Numerous circulated examples exist, but Mint State examples are scarce. PCGS has certified nine Mint State examples (as of May 2011), the finest of which is a single MS-66. The PCGS Condition Census includes grades from MS-62 to MS-66.