The underside of the arm that holds the gap is marked “Apollo E.P.B.M. Made in USA.” EPBM is the abbreviation for Electro-plated Britannia Metal
From Wikipedia: Britannia metal (also called britannium or Britannia ware) is a specific type of pewter alloy favored for its silvery appearance and smooth surface. The composition by weight is typically about 92% tin, 6% antimony, and 2% copper. Britannia metal is usually spun rather than cast, and melts at 255 degrees Celsius.
Ths flask appears to date c. 1920 judging from the dress of the golfer and caddie, and the bag in use and nature of the flask itself.