Set of 4 industrial Thur-op-seat chairs with armrest

Vintage chairs with an industrial design and armrest ( removable). The design can be traced back to the famous Pyramide chair by Wim Rietveld. The chairs has a brown metal frame, chairs are marked at the back: Thur-op-seat. The condition is reasonable given the age, there are some visible signs of use (see detail picture's). The size is 80 cm high, 47,5 cm height and 53 cm wide.