FK.51 intermediate trainers, army air force serial numbers 10, 15 and 18 through 21, in flight over Holland. The first batch, s/n 1 through 10, were painted brown; the second batch were pale blue and the typeface of the serials was also changed. The pale blue machines here were all delivered on 9 February 1938. A fatal collision involving s/n 20 dates the photograph before 15 July of the same year. Photo from: Nederlands Instituut voor Militaire Historie