An overdate occurs when a coin exhibits dates of different years. This can occur when the date is punched into a die for one year and the date of another year is punched over one or more of the digits of that working die. The intentional reasons this could occur are: 1) A shortage of dies or metal to make dies. 2) The mint may have made too many dies for a particular year and had many left over. Conceivably, an overdate could also have happened by accident. The engraver could have picked up the punch for one year and when he went to strike the date punch a second time he picked up the punch for a different year.
The 1888/7 is the only clear-cut overdate of the Indian cent series, with the last 8 punched over a 7. It's a difficult variety to recognize without magnification, especially on well circulated examples. The most prominent trait is the lower nub of the 7 that sticks out below the lower left portion of the 8. Less obvious are the upper left serif of the 7 above the 8 and remnants of it in the upper loop. A micro close-up of this finest known also shows the vertical stroke of the 7 where the loops of the 8 meet.
1888/7 Overdate S1 PCGS MS64RB Finest Known(Closeups of date on next page)