Might this have been some private arrangement with a friend in Portugal to forward correspondence between Belgium and the U.K.? The clues to that are pretty good. There is no listing for the address 34 Rua Ernesto da Silva in a Lisbon suburb in the reference "Undercover Addresses of WWII – Third Edition" by Charles Entwistle, and he confirmed that in a recent e-mail. There is nothing unusual about the building currently at that address which appears to possibly be the home and location of a plumbing and electrical service person, without any signage in Google map/street photos.
And then there is a bit of "education" in European penmanship here. What is the addressee's name? First noting between the 2 covers, apparently by the same hand, the "L" in Lisbon. It is very suggestive of a name Gabriel "Leydel". The "a" and "d" in "da Silva" of the address explain the "odd looking" letter "d" in "Leydel" – not the penmanship strokes used in the U.S, but consistent and neat in any case...and a European style sometimes seen elsewhere. However, while a common enough name in a Google search, but not with clear Belgian ties, that went nowhere. Trying again, and looking at the strokes of that "L" of "Lisbon" seemed quite convincing that it was not exactly a match to the name, but rather to the "S" in the street name "da Silva". The amazing result was, via Google, the Belgian ties of one quite noteworthy WWII RAF pilot, one Gabriel "Seydel".
One thing I liked about these two covers was that the name was underlined – for example in red, and also in red was an added number "349", and the "349" was on both covers. This had suggested to me a strong possibility of the forwarder in Portugal working off a card file or notebook of addresses for mail forwarding, and this addressee was number "349". Recently I had written an article for the Scandinavian Collectors Club "Posthorn" magazine (May 2016, cover and pages 13-19) about such a card file used by the Norwegian Embassy in Lisbon for mail routing between Occupied Norway and many escaped Norwegians especially one's tied to the Government in Exile – and the Embassy person who oversaw this effort frequently used various residential addresses in hopes of not attracting German censorship attention. Maybe the Belgians did something similar? That was the thought.
Anyway, back to our addressee Gabriel Seydel. The first result was 100% in French, and with a lot of military abbreviations that meant nothing to me. It was clear there might be a link, though. Refining that search now produced a fascinating biography, and even a photo! That information is shown below . And the key item -- the greatest key assuming it was put on the cover at the time of its original handling -- is that number 349, and being on both covers! That turns out to apparently have been used to direct the addressee's mail to his RAF Squadron of Belgian Volunteers, Squadron 349! As an aside, our addressee bounced between Squadron 350 and 349 during the war years, but the mail sorter here had it right for the time of these letter mailings.
Biography of Gabriel Seydel
Above, a biographical piece about Gabriel Seydel from the website http://www.350sqn.be/biography/Seydel%20Gabriel.htm The website is dedicated to information about RAF Squadron 350 and those who served in it at any time during the war -- which he did for a time.