The letter above is very interesting.
It was sent by air mail from Basel (Switzerland) to a recipient i Malmö (Sweden).
When I look at available airline time tables from the time it appears to me that the logical route would have been Basel-Mannheim-Frankfurt. Then Frankfurt-Hannover-Hamburg-Copenhagen-Malmö. Airline would have been Deutsche Luft-Hansa all the way.

BUT, on the envelope there are cancellations implying that the letter has passed through Stockholm (Sweden) using the trial flight conducted between Stockholm-London. It shold have reached Malmö on its way to London as Malmö is indicated as an intermediate airport ... But it also have a cancellation implying it was never unloaded in Malmö, but instead reached London and through the overnight service ended up in Malmö on the return flight from London.
However, to reach Stockholm from Basel I can't see any other option than using Deutsche Luft-Hansa's seaplane from Stettin via Kalmar to Stockholm. If so, I can't how this was achivable in just one day (4th of Sep to 5th of Sep)???

MYSTERY!
I just realized another option. Basel-Paris-London with Imperial Airways (one day). The using the trial night flight London-Stockholm which as mentioned had a stop in Malmö. I.e. the letter never passed Stockholm. Obviously I have answer why the cancellation LUFTPOSTEXP. N:r 1 is on the envelope (an answer I cannot give today). Well well ...

In addition the postal office never could identify the reciever in Malmö, so the letter was apparently sent back to Switzerland.

***

The cancellations seen on the address side of the envelope are:
1) Cancellation in Basel Flugplatz on Sep 4 1928
2) Cancellation LUFTPOSTEXP. N:r 1 on Sep 5 1928 (stating "Femte turen" which means 5th flight, of the Stockholm-London trials).
3) Cancellation AIR POST OFFICE LONDON-STOCKHOLM OVERNIGHT SERVICE 5-6 SEPTEMBER 1928.
4) Adresse insuffisante Adressen ofullständig. Not dated (since it's also a Swedish statement I belive it's stamped in Malmö).
5) Retour á l'expéditeur.

The cancellations seen on the backside of the envelope are:
1) Arrival cancellation in Malmö on Sep 7 1928.
2) UTAN NÄRMARE ADRESS OBESTÄLLBART (which means: Address insufficient for delivery).

This letter was kindly given to me by Peter Bamert in Switzerland.

We can, at least partly, follow the route of this postcard took just 3 months prior to the brake-out of WWII 1939.
It was sent by airmail (night flight) from Stockholm (Sweden) to Hannover (Germany). Cancelled in Stockholm May 23.
I have tried to figure which aircraft ABA was using in May 1939 for its night mail flights from Stockholm.
I have two candidates. Either SE-AEH Västergötland, a Junkers W34, or, SE-BAE Svalan, a Junkers Ju-86. Both planes were used up until 1940 when the flights were cancelled due to the ongoing war.

The postcard was then transported from Hannover (Germany) to Marseille (France). Cancelled in Marseille May 24 (i.e. the day after).
The exact route between Hannover and Marseille can not be determined.

It left Marseille (almost certainly on a flying boat) and arrived in New York on May 27 (cancelled 5.30 PM).
Two days later in was cancelled in Rahway NJ 6.30 AM.
The exact way of transportation between New York and Rahway can not be determined.
Since Rahway is just around 5 kilometres west of Staten Island I guess it was a land transport ...

The postcard was most likely sent on Pan Am's FAM-18 southern route:
Marseille (France)-Lisbon (Portugal)-Horta (Azores)-New York (USA)
[FAM=Foreign Air Mail route]

I eastbound route was inaugurated on May 20, 1939.
The return (westward) flight left Marseille on May 25. It left Lisbon on May 26, made a stop at Horta, leaving Horta the same day.
Finally arriving at New York on May 27.

The Boeing 314 used (named Yankee Clipper) was piloted by Arthur E LePorte.

[Source: www.aerodacious.com]

Follow this link to see my collection of Pan American Clipper Ships.