In 1863, the U.S. Mint produced only 550 Proof Three Cent Silver pieces. This was a substantial mintage for the time, as there was a limited number of collectors that focused on obtaining American coins during the Civil War. However, to the number of active collectors today, this is an extremely small supply. As a result, this date is always popular and in-demand.
Of the examples certified by PCGS, the majority are either PR63 or PR64. Gems are really hard to find and PR66 appears to be the terminal grade for non-Cameos. Cameo examples are almost six time as rare as non-Cameos. This is the only Civil War-era Trime that is unknown in Deep Cameo.