(This is needed for a factual report and as such, is worded
accordingly:)I am an American mother of an American boy who just
volunteered with the 50th Northumberland Fusiliers in England,(their
HDQ was Darlington in Yorkshire, England,) during World War Two, and I
want to write to him in 1940 or 1941. I know their actual physical
address is Reed Court, 7, Harewood Hill, Darlington...BUT, I
understand that with British censorship, special codes needed, army
division corps, regiment info, etc., how exactly would the envelope
address be worded? What was the military postal address for the
Northumberland Fusiliers? Her letters would not have been sent to a
direct location but a drop off place to be forwarded. Any help greatly
appreciated.

Hi Lentionne - Have you already written to the Northumberland
Fusiliers Museum to see if they can help?
http://www.northumberlandfusiliers.org.uk/research.html

You could try asking the Imperial War Museum. 
I found them very helpful with a recent query, just using their
'general email' address:
http://www.iwm.org.uk/corporate/contact.htm

This is their North of England page with more London contacts at the
bottom:
http://www.iwm.org.uk/north/contact.htm

Good luck - I hope you can find the answer - Leli