An overdate is a variety in which you can see the remnants of a previous year's date underlying the date the coin was minted. In such cases, an obverse die from a previous year was modified by having the date polished away. The new date was stamped in, but elements of the old date remained. Overdates exist in the Morgan series, but they are few and far between and concentrated in certain dates.
An overdate is distinct from a "doubled date." A doubled date simply shows doubling from where the date was stamped into the die. An overdate is one date "over" a different date entirely.
Most of the overdates in the Morgan series occurred in 1880. For all four mints there are 1880 over 1879 varieties. The most dramatic is the 1880-CC VAM-4, a Top 100 VAM. 1880-P,O, and S Morgans also have overdate varities. Well-recognized overdates are similarly found on 1887-P VAM-2 and 1887-O VAM-3 varieties, also Top 100.
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