Among the fifteen varieties known of the 1820 Large Cents, three are Overdates. All three come from different dies. Newcomb 1 is known as the Large Overdate (referring to the size of the date); Newcomb 2 and 3 are Small Date Overdates. PCGS recongizes any of these three varieties as overdates.
Of all the 1820 Major Variaties, the Small Date is the least common, followed by the 1820/19 Overdate. The most common Major Variety, by far, is the Large Date.
Among the Overdates, Mint State examples are very scarce. PCGS (as of October 2011) had certified 17 Mint State Overdates, the finest of which is a PCGS MS66 Red-Brown.