Airmail to America during World War II

One of my stamp collections involves gathering impressions of advertising flags from stamping machines. Specifically, those flags from just before, during, and just after the Second World War. As an example, an impression of the stamp flag: "Vacation in a Peaceful Fatherland," used in a stamping machine at the Post Office dispatch center in Amsterdam Central Station. An impression dated May 7, 1940, at 11:00 PM, just a few days before the invasion of our country by German troops. During the occupation, many types of advertising flags were used. For example, I found an airmail envelope with an imprint of the stamp flag: 'Have You Already Donated to Winter Relief in the Netherlands?' A stamp flag used in a stamping machine in The Hague. The airmail letter was postmarked March 25, 1941. Two copies of the 22½ cent 'barred stamps' were affixed as postage stamps, a franking not encountered every day. On the left side of the letter, the sender had written: "by clipper." The letter was destined for the United States, Washington. Due to the war, there were no direct postal connections between Germany and the occupied territories with the United States. Therefore, an arrangement was established to have mail shipments to that country transported via Portugal. To this end, the German postal service maintained an air connection with Lisbon using Ju 90 aircraft. Such an aircraft is depicted on a stamp of the Grossdeutsches Reich from 1944, even though the model is shown in a 'streamlined' manner, as the photo illustrates. Other designations also occur, such as 'Transatlantic Airmail', 'by transatlantic clipper', and 'par Lisbon' or 'via Lisbon'. Why via Lisbon? Portugal was not occupied by German troops and remained neutral. As a result, both the German Lufthansa and the American Pan American World Airways were able to use the facilities in Lisbon to land aircraft. Lufthansa used the brand-new Portela Airport. Pan Am, however, used seaplanes, and therefore a new airport for seaplanes was opened simultaneously, approximately three kilometers from the other airport. This airport, named Cabo Ruivo, was situated on the Tagus River. A special road, the Avenida Entre-os-Aeroportos (now the Avenida de Berlim), was constructed between the two airports. The American airline used Boeing 314 aircraft. This seaplane was specially designed to make transatlantic flights. In total, the company had 12 aircraft of this type. They were called 'clippers', with names such as the 'California Clipper', 'American Clipper', 'Anzac Clipper', 'Dixie Clipper', and the 'Yankee Clipper', which crashed in Lisbon on February 22, 1943, after the 241st transatlantic flight. But what was the transit route of the letter from The Hague to Lisbon? Many airmail letters destined for the United States had no markings. It was common, however, that the letters were always opened by the Censorship Service to check the contents. The letter I have included in the collection also has such a strip. This strip was affixed to the back and pasted over a German stamp impression. The only letters visible were: KÖLN-DEU. The airmail letter had therefore been transported from The Hague to Cologne and stamped there. Using a special technique, I was also able to read the rest of the text in the stamp: KÖLN-DEUTZ. Deutz used to be an independent city on the other side of the Rhine, opposite Cologne. However, it was on the side where the nearby airport was located. And with that, the postal route was clear: posted in The Hague on March 25, 1941, stamped in Deutz on March 27, 1941, and transported via Lisbon (no transit date known) to Washington (no arrival date known). Why the few other airmail envelopes I have seen did not have an impression of the KÖLN-DEUTZ stamp is unclear to me. Or was this important airport for postal transport supposed to remain 'unknown' due to wartime conditions? Was that why the censorship strip pasted over the impression? Anyone who knows the answer is welcome to share it! In any case, stamp impressions give me great collecting pleasure, even from dark times.