I received an email and photo from Jack Herris from the League of WWI Aviation Historians who said:
The Curtiss HS-2L was converted from the earlier Curtiss HS-1L by inserting a 6-foot wing panel between the wing center section and outer wing panels to increase wing span and area to enable the aircraft to carry a heavier load. The “L” in the aircraft designation indicated use of a Liberty engine, a V-12. The engine is not seen in the photos but only the prototype HS-1 aircraft was tested with other engine types, all the production aircraft using the V-12 Liberty. Unfortunately, there is no full image of the aircraft to enable verification of the wing span and number of bays of struts; the HS-2L had an additional bay of bracing struts compared to the HS-1L. Both types of flying boats were used operationally from seaplane bases in France during WWI for patrols, especially anti-submarine patrols.