Most Martin guitars made between about 1885 and the beginning of serial numbers in 1898 are dated under the top of the guitar in pencil or blue crayon. Occasionally initials are also seen under the date.
This guitar is one of the few instruments that are dated prior to 1885. In this case the guitar is dated "AD 1875" (AD indicating "Anno Domini" and does have initial under the date although they are very hard to read. Martin only had four employees in 1875: C. F. Martin Jr., C. F. Hartmann, Henry Goetz and Reinhold Schuster. The signature could be "Goetz" but closer examination will be needed to confirm this.