From what I can gather your button would date between 1859 and 1868. That specific backmark was used from 1859 until 1893. However. the "Map on Tree" was replaced by an "Eagle on Shield" in 1868. The "Map on Shield" returned in 1939 but that was 46 years after your backmark went out of use. I would guess that your button was made between 1859 and 1861/62 because I don't think Horstmann supplied buttons to the Confederacy but I could be wrong about that. Also, if Horstmann resumed supplies it could date between 1865 and 1868.