This info is from another post that talked about the different FTR coins and why they were struck. This galley FTR was issued for a specific reason- it seems to be a reference to a visit Constans made to Britain in A.D. 343, so he was celebrating some sort of a victory on these coins. Whether it was an actual victory or a moral victory is not known. His brother Constantius II issued coins celebrating a dubious victory-- the two captives FTR was issued to celebrate the victory of the Roman Army under the command of Constantius II at the Battle of Singara, which occurred in A.D. 344. The Roman Empire claimed to have defeated the Sassanid Army led by Shapur II, but in reality the Persians left the battle, as Shapur II realized he was spread too thin.
For more see RIC VIII and the article by Konrad Kraft “Die Taten der Kaiser Constans und Constantius IIâ€