Thur-op-seat chairs with armrest set of 4

Vintage chairs with an industrial design. The design can be traced back to the famous Pyramide chair by Wim Rietveld. The chairs has a grey metal frame, special about these are the armrest. The chairs are marked at the back: Thur-op-seat. The condition is reasonable given the age, there are visible signs of use (see detail picture's). The size is