W. P. Hollister was promoted first lieutenant.

The evidence

• “S. Hollister (promoted Second Lieutenant June 13, 1862, First Lieutenant November 28, 1864), W. P.”

  1879-history-of-rock-county/12-rock-county-ch06-politics-war-record.md, PDF p. 462 / book p. 466

  Butterfield, C. W., et al. The History of Rock County, Wisconsin: its early settlement, growth, development, resources, etc. Chicago: Western Historical Company, 1879. Cornell University Library via the Internet Archive, archive.org/details/cu31924028871585.

  Grade A: Found on a page of one of our sources

  

• “S. Hollister (promoted second lieutenant June 13, 1862; first lieutenant November 28, 1864; captain February 15, 1865), W. P.”

  1908-brown-rock-county/16-military-history.md, Vol. 1, PDF p. 375 / book p. 345

  Brown, William Fiske, editor. Rock County, Wisconsin: A New History of Its Cities, Villages, Towns, Citizens and Varied Interests, from the Earliest Times, Up to Date. Chicago: C. F. Cooper & Co., 1908. 2 vols. archive.org/details/rockcountywiscon01brow, archive.org/details/rockcountywiscon02brow.

  Grade A: Found on a page of one of our sources · may repeat 1879-history-of-rock-county, so not counted as independent