When the Union army built Fort Curtis in 1862, the 34-star flag flew over the fort.
The day the Battle of Helena was fought, July 4, 1863, the 35-star flag the official U.S. flag. The new star represented West Virginia, admitted to the Union on June 20, 1863. This is the flag that flies overhead.
The Union army continued to man Fort Curtis after the Civil War so it also flew the 36-star flag, which became official on July 4, 1865. The new star represented Nevada, admitted to the Union on October 31, 1864.
These flags are the official style adopted by the military. Until 1912, neither the order of the stars nor the proportions of the flag was prescribed. Flags with the stars arranged in a circle, star and other patterns were common.