I believe the mystery ship to be USS Lexington, CV-2, built by Bethlehem Steel at Quincy, Massachusetts. She was laid down on 8 January 1921, and commissioned as CV-2 on 14 December 1927. The photograph shows her on the Panama Canal and possibly dates from early 1928. She had passed into the Pacific by mid-1928 and ran trials off Southern California, achieving 34.59 knots while producing 202,973shp. She then did the San Pedro Sound to Hawaii trip at an average speed of 30.7 knots, to join the Pacific Fleet. By 1929 she was at Puget Sound Navy yard.
Lexington was similar to USS Saratoga, and both passed through the canal on several occasions, but the vessel in the photograph does not exhibit the differential funnel markings applied to the class after the shambolic Fleet Problem 1X exercise of 1929-30, so it must be Lexington. The only difference I can see between the two ships is in the arrangements of the forward tower superstructure above the range finder. Lexington was lost on 8 May 1942 after being hit by torpedoes at the battle of the Coral Sea.
Sandy McAuslan, Renton