What is (or was) the "international frequency"?
The international distress frequency has been 500 kHz, since 1908.
Copious details are at https://en.wikipedia.org/wiki/500_kHz.

During the war, this frequency was used for reporting distress and for monitoring distress, by the Germans and the British.  It was also used before and after the war for this purpose, by many countries, by aviators and mariners and others.