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 Above type is approximately four times as rare as the Mintmark Below type.
As with the 1875-S Mintmark Below Dimes, many of the Mintmark Above appear to have been struck from worn dies. This gives the appearance of weak details that simulate wear. Knowlegeable collectors understand the vagaries of die states, but new collectors prefer well struck examples.
Researcher Gerry Fortin has identified 6 different die varieties for this type (compare to the 22 die varieties of the 1875-S Mintmark Below).