What does HS stand for when stamped on the muzzle end of a 1903 springfield rifle?

HS on the muzzle end of a 1903 Springfield rifle stands for "High Standard."

This marking indicates that the rifle was manufactured by the High Standard Manufacturing Corporation, a company that was contracted by the US government to produce rifles during World War II. While the Springfield Armory produced the majority of the 1903 rifles, High Standard and other companies like Remington and Winchester also contributed to the war effort.