On the occasion of HAFNIA 87 international stamp exhibition held in Denmark, Cambodia issued a set of 7v and S/S thematic stamps featuring helicopters.
Above shows one of the FDCs of the full set of three. What makes it stand out is the special cachet right below the 1.5r stamp. This particular cachet was available only at the exhibition venue.
"Kampuchea" (name of Cambodia at the time) is never a popular country to collect for Danes. So I guess the one who bought this FDC set at the venue might just be a thematic collector.