British tanker San Conrado visited the Black Sea according to a letter of the German naval attaché at Istanbul dated 24 February 1940

In his letter dated Istanbul, Turkey 24 February 1940 No. 774 g. reported the German naval attaché to the Oberkommando der Wehrmacht Abteilung Ausland the neutral ships passing Istanbul coming out of the Black Sea and loaded with contraband for or chartered by the enemy. An item reported on 18 February 1940 the British tanker San Conrado armed with not only a gun but also a flak at the stern.

Source

Bundesarchiv. German Marineattaché for Greece, Romania and Bulgaria 1939-1941. RM 12-II/459.