Captain Wolfgang von Gronau (Scott #C36-45). 25,305 sets were issued, and Scott '14 prices the unused set at $29.- ($44.25 for NH).
The set has potential multiple market appeal among collectors of Philippines, U.S. Possessions, Aerophilately, and in Germany, due to interest in von Gronau. While I sometimes recommend obtaining expertization when buying overprinted stamps, this set is an exception because it's too inexpensive to fake.
As a newly democratic country of 92 million which is moving away from from its centuries-old complete dependence on agriculture, the Philippines could turn out to be one of the most successful emerging markets in the Pacific Region. The government tends toward fiscal conservatism coupled with long-term economic planning, and annual GDP growth has been around 6%-7%. Barring extreme political instability, it is likely that the Philippines will be one of the fastest growing economies over the next decades.