At the 1953 Haarlem Improvisation Competition, the contestants were Paul Barras, Anton Heiller, Piet Kee (winner), Matthieu Prange and Karl Richter. Adriaan Engles provided the themes and asked for a Sonata in three movements according to the instructions shown with the themes. Thanks to the NCRV archives it is possible to hear the improvisations of the finalists (Anton Heiller, Piet Kee and Karl Richter) through the links below:
Many other improvisations and performances are available through the NCRV archives here.