I found it difficult to research this cover but I did find the following, which, while it does not mention this flight in particular does give some insight to the history of aviation back in the 1930s in the region.

The history of commercial aviation in the Philippines started in 1925. The first regular air services were launched in Iloilo (Central Philippines), said to be the birthplace of Filipino commercial air transportation. Jose Tinsay, an Ilongo aviator, was the first to fly the 43-kilometer Guimaras Strait between Iloilo and Bacolod.

In 1932, Don Eugenio H. Lopez, Sr., the sugar and shipping magnate, launched Iloilo-Negros Air Express Company (INAEC) which became the first Filipino-owned private airline in the Philippines with its operational base in Iloilo. INAEC’s first aircraft, a Stinson Tri-Motor, had its inaugural flight on February 1, 1933 from Iloilo to Manila. Regular air services between Manila, Bacolod, Iloilo and Cebu started within a year, and then expanded to Zambonga and Davao in another two years.

I can only assume from that report that this cover was carried on a Stinson Tri-Motor. And, that the flight to Negors was a non event given that I can find no reference to it. Perhaps that is what I find interesting about this cover.  The airline,(INAEC) eventually became part of what is today Philippine Airlines.

Also, hard to believe that the stamp, SG 451 issued in 1933 has a catalogue value of only £0.25. Still, it is a lovely cover to add to my aerophilately collection.

Best wishes.. Michael