The various Sudan literature that I have makes no mention of an Express Mail service around this time. Given the stage of the war I would not expect any form of Express Mail service. It would have just been carried on the Horseshoe Route.
Another curiosity with the cover is the 3½ piastres franking. The air mail fee should have been 4½ piastres - underpaid, so no extra payment for any express service.
From a bit of googling, I think the 'Express' label is a British one.
Postmarked Khartoum - looks like 7 March 1941.
Back-stamped Devlali 12 March 1941.
I anyone has any information about an Express Mail service, I would be very grateful.
[Today's trivia - According to wikipedia, the British Army camp at Deolali/Devlali gave us the term "doolally".
https://en.wikipedia.org/wiki/Deolali - It is also the source of the British slang noun doolally tap, loosely meaning "camp fever", and referring to the apparent madness of men waiting for ships back to Britain after finishing their tour of duty. By the 1940s this had been widely shortened to just "doolally", an adjective meaning "mad (insane)" or "eccentric".]
Thanks - Dave.