With 2 stamps "1968 Christmas" on the back of the envelope

This card is a "QSL-card", i.e. a card that radio-amateurs sent to each other to confirm a radio contact or simply to say hello to each other.

This one was sent to my father Félix Duval (1914-1991, call-sign ONL 362), who was a pioneer of amateur radio, here in Belgium.