A contactadvertentie is a contact ad, an advertisement by a person looking for a spouse. The ads are almost always anonymous, but give you some idea what people found important in a spouse.
This ad from the Rotterdamsch Nieuwsblad [Rotterdam Newspaper] of 14 January 1913 shows one woman using an ad to find a spouse.
A capable young woman age 25, with a girl age 7, wishes to make her acquaintance with a widower, no older than 30 or 35, from the working class. Letters delivered free of charge are expected until Tuesday afternoon next at 1 PM under number 5402 office of v.d. Bl. […]ken.