Many a counterstamped coin was holed with some intent ....

John Zettler is listed in the 1872 NY City Directory at 88 Norfolk Street. His business was therein was simply described as "guns." Previously, in the 1850 Mercantile Directory, he was listed as a gunsmith in NY County.
Thanks to online research, I was able to gather some interesting history about John. Born about 1830, he arrived in the U.S. in 1837. His family was from Marburg, Germany. An accomplished gunsmith, John made a pepperbox pistol that was later patented by Blunt & Syms. A mark that's identical to this counterstamp appears on an 1850's era large bore rifle that John made.
During the Civil War, John enlisted in Company H of the NY Third Infantry on 7-13-1863. He was appointed as artificer or weapons' craftsman in the field. He was mustered out in Washington, D.C. on 8-22-1865. Following the war, John and his brother had businesses at various addresses in the Bowery. The Zettlers produced percussion and breach-loading target rifles. Many Germans in those days formed shooting clubs and held contests for marksmanship.
As this host coin is dated 1865, John likely applied his gunsmith mark during his days at the Bowery shooting gallery. To date, this is the only example of the Zettler counterstamp, listed by Brunk or Rulau.
Here's a link to the Zettler Bros. photo gallery:
http://www.pbase.com/halp/zetter_bros&page=all