How Did This Cover Make Its Way By Air To Northern Rhodesia In 1941/1942?

Hi there! I would appreciate if someone is able to enlighten me on how this cover made its way to Northern Rhodesia way back in the WW2 years of 1941/1942.

Postmarks on the front:
Dec 19, 1941 Philadelphia
Jan 24, 1942 Northern Rhodesia

Postmark on the back:
Jan 14, 1942 Leopoldville

The cover came with the following note:

Cannot be by FAM-22 Route 6 - first eastbound flight #6001 departed Dec 6 1941 and arrived at Leopoldville Dec 12 1941 (too early); second eastbound flight #6003 departed on Jan 18, 1942 (too late)

Cannot be by Special Mission flights - first eastbound shuttle carrying eastbound mail from the US was SM10, which is flight #6003.

Thanks!

In addition to FAM 22, there were apparently hundreds of charter flights that were classified and which carried lots of mail — the SMs. The note you have indicates that this cover was not SM, but you might want to check if that conclusion is correct.
As a starting place, Gordon Eubanks' slide deck at. https://www.collectorsclub.org/wp-c...ay-21-v2.pdf. He includes a citation further along in the deck.

Hope this helps or is at least of interest….