Today I found a new picture of Johannes Blok: it is featured in the documentary “Dutch Wings over Jackson”. Johannes is the man on the front row right. If anyone knows the names of the other Dutch pilots, please let me know!
The documentary is well worth watching - see
https://youtu.be/preyJJl1Xws?t=537
Seeing the white tropical uniforms and Warrant Pilot rank insignia, I think this picture was made during their training period in the Netherlands East Indies.