15mm, Civil War General Staff Officer cuff button. "Horstmann/New York" ("flat back" as seen on the Civil War AK1Av and KY1Av). This backmark is a good solid Civil War mark (one that I have not seen occur on any post war uniform button).  Enjoy this nice Civil War Officer's button.  It would have been used on the vest, cuff, or kepi of a Civil War Officer (also, could have been used on a militia officer's epaulette).