What is (or was) the "international frequency"? I have recently read a book about Douglas BADER. This pilot has lost his legs (because of accident in 1931) but after (thanks to his single-mindedness) he takes up the "broomstick" again and became a ace with ~30 victory...
After he is shot and captured by the enemy: the Germans. Germans contact the UK in the "International frequency" to send prostheses. The Germans propose a deal but the RAF drops it during a bombing raid with 11 messerschmitt shot in the process.(at the same time)

What information are available about this frequency? (name, usage, frequency).

Have you question about this story?
Bonus :
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.