Set of four chairs 1960 Netherlands

Set of four vintage school chairs from the 1960s from the netherlands. These seats are rarely offered. This is an unknown model that looks a lot like wim rietveld's pyramid chair. The chairs are in good use with normal traces of use and age, see photos. There's a ceiling missing from a single chair. The chairs are stackable. Dimensions: 47 cm long, 43 cm deep, 79 cm high and 47 cm high seating.