The term 'plumed military cap' is a reference to the SHAKO, a type of hat that was popular among European and American military forces during the 19th century. The SHAKO was typically made of felt or leather and featured a tall, cylindrical shape with a plume on top. It was often decorated with metal ornaments and was worn by both officers and enlisted personnel. The SHAKO was eventually replaced by the more modern peaked cap in the early 20th century.