In 1875, the Philadelphia Mint experimented with two different mintmark placements for the branch mints at San Francisco and Carson City. One version has the mintmark placed normally, below the wreath. On the "odd" version, the mintmark is placed within the wreath in the field below the word DIME. The reason for the experiment is unknown, and may just be a misplacement of the mintmark by an employee unfamiliar with the history of the series.
According to the PCGS Population Report, the Mintmark Below type is approximately four times as common as the Mintmark Above type.
Some examples of the 1875-S Dime (Mintmark Below) were struck from worn dies. Therefore, date and type collectors should focus on finding well-struck examples.
Researcher Gerry Fortin has identified a whopping 22 different die varieties for this date.