According to the WI State Death Index 1859-1979, Erwin F. Lenk died 13 Oct 1864 in Milwaukee, Milwaukee Co. at age 59.
The link: http://www.vitalsearch-ca.com/gen/wi/_vitals/wide59_t.htm
Type your surname in the box at the bottom to go to the fiche page. Check the index for the correct fiche square or you can hunt and peck.
Hope this helps.