PK-SAU was the first of two W-4s, designed by Dutch engineer Laurens Walraven and built by him and army air force personnel at Bandung-Andir airfield as it was then known. The other W-4 was PK-SAI. It was registered to the Nederlandsch-Indische Vliegclub Bandoeng (Netherlands East Indies Flying Club Bandung) in 1938. Note the KNILM Lockheed 14 in the background on the left and the Fokker on the right. Photo from: Walraven family album

This looks like PK-SAI; its sister aircraft PK-SAU had a brighter paint job as well as a marking on the fin, which this aircraft doesn't have. It looks like an engine test. Photo from: Walraven family album

This is probably PK-SAI, the second and last Walraven W-4, which first flew in 1939 and crashed in December of that year. It is unclear why the registration has been removed from the aircraft. Photo from: Walraven family album