I have had these four chairs for around 15 years.
They are 'Rhondo' Chairs by Jan Lunde Knutsen - for Karl Sørlie & Sønner Sarpsborg of Norway. They are in finished in black vinyl and chromed steel.
Good vintage condition - a provocative seating design in a semicircular formation with vertical chromed tubes composing a highly detailed and innovative frame finished with a c-shaped backrest, Scandinavian Modern influenced by the Bauhaus.
The only similar examples I can find online, are selling in America for nearly £950 each.
The only obvious signs of age I should mentions are - on one of the chairs, where there is more wear to the seat area than the others, with the vinyl displaying uneven tone and surface condition. There are a few nicks and areas of wear showing on the vinyl of the four pieces, as you'd expect. There is a more significant area of wear on the right hand arm of one chair, where the vinyl has spilt and you can see the foam beneath - about 25mm x 20mm. The chrome is tarnished in certain areas across the four chairs too. None of this spoils the unique style and overall great vintage condition of the chairs - I only mention it to be honest and to avoid any time wasting.