Source: Supermarine Southampton Flying Boat - Nightime 'Landing' (YouTube)
It's a standard flare bracket – controlled from the cockpit,:76 it holds a burning flare to illuminate the water surface at night when landing.
This particular streamlined design is most likely based on the 1922 patent by Holt Harold Edward Sherwin. It's a clamshell device; when open, the two halves help reduce the heat going up. Those flares are also re-ignitable. The earliest design, also by Holt, was filed in 1917. They are also known as Holt flares.
See item 18 in the front and lower-wing views above.
Provision is made on the rear spars of the bottom outer main planes for the attachment of the standard flare brackets. The flares are controlled by push buttons on the port side of the pilot's instrument board.:76 [emphasis mine]
The Air Navigation Directions (...) require aircraft to be equipped with landing lights (lamps or wing-tip flares) for flights by night (...) The construction of the flare brackets and their position on the wing of the aircraft must be such as to prevent the flame from the flare overheating the wing or aileron fabric when the aircraft is in flight or standing on the ground. [emphasis mine]
Source: twitter.com; cropped
In the crop above, the object at the leading edge is a navigation light, the one below at the trailing edge is the flare bracket.
(My guess, with the engines near idle for landing, there wasn't enough electrical power for bright lamps, or they were heavy and/or unreliable back then.)
1: Great Britain. Air Ministry. (1929). The Supermarine Southampton Boat Seaplane, Mark II. (Metal Hull) Two Lion Engines ... Air Ministry. Issued November, 1929. London: H. M. Stationery Off.. (hathitrust.org)
2: (1939), "A German Long‐Range Flying‐Boat: Some Particulars of the Latest Dornier Seaplane to be put into Service on the South Atlantic Route", Aircraft Engineering and Aerospace Technology, Vol. 11 No. 9, pp. 357-369. https://doi.org/10.1108/eb030544
3: Sherwin, Holt Harold Edward. "Landing light or flare for attaching to aircraft." U.S. Patent No. 1,423,326. 18 Jul. 1922.
4: Sherwin, Holt Harold Edward. "Flare for use on aircraft." U.S. Patent No. 1,360,527. 30 Nov. 1920.