Official caption: A blue jacket aboard Pennsylvania telling two feminine visitors all about the mechanism of the anti-submarine guns, December 1918.
Photo by Underwood & Underwood via the National Archives 165-WW-332D-42
The gun looks to be a low-angle 3-pounders (47mm), common on earlier battleships of the 1900s for use not only against oncoming torpedo boats but also submarines who, more often than not, were encountered awash or surfaced except during their final attacks. By 1918, they were more or less just used as saluting guns, which would jive for how the two above guns are mounted (i.e. side-by-side so that one crew could work both guns to keep up an easy cadence while the close mounting made it next to impossible to traverse or elevate).
USS Pennsylvania (BB-38), commissioned 12 June 1916, was originally completed with four 3-inch high angle guns for use as “balloon busters” and anti-aircraft artillery, her smallest “usable” guns.
RADM Henry T. Mayo’s flagship in 1917, the battleship cooled her heels off the Atlantic coast during the Great War, only heading to France in early December 1918 to escort Mr. Wilson to Paris. She then escorted the coal-fired dreadnoughts of Battleship Divisions Nine and Six back to New York, which had served with the British, arriving back home on Christmas night to a big celebration.
Her Second World War would be much more exciting.