Dox Covers - Netherlands To NY - 9 Month Trip?

I don't collect First Flight covers but my world-wide collection has a hole that a few of these stamps could fill! Of course before floating anything I looked a bit closer at these flight covers and found the following pretty interesting.

The Dornier DOX was developed by Germany in 1929 and as the largest (and heaviest) 'flying boat' ever produced. It broke the world record for carrying the most people on board (169 soles) on 10/29/1929. As a means to publicize the plane to the American market, it took off from Friedrichshafen, Germany on 3 November 1930 for a transatlantic test flight to New York. The route went from Germany, to Netherlands, England, France, Spain, Portugal, down the west coast of Africa, across to Natal, Rio de Janeiro, and then up the east coast of the US and finally to N.Y.


Unfortunately while still in Europe (Lisbon) a tarpaulin came in contact with an exhaust pipe and burned much of the port side wing off. It sat in the Lisbon harbor for well over a month while repairs were made. It finally landed in New York on August 27, 1931; taking a better part of a year to make the trip.

I assume that these covers were mailed in Netherlands and made the long trip to NY. Any additional info/corrections to my limited knowledge is welcome.
Thanks in advance
don

Mr. Hekker was a stampdealer in Amsterdam, mostly between the two world wars.
That is the extent of my knowledge of these covers!

Peter