The book "The G-Plan...
The book "The G-Plan Revolution" cites the chair as G-Plan's "enormously successful 'Saddle range'", which was introduced in 1969. I have seen many references to this chair, or a very similar one, being designed by Ib Kofod Larsen. Not sure of the validity of that though.
The High Wycombe Furniture...
The High Wycombe Furniture Archive has many of Kofod Larsen's technical drawings available online, and there are enough similar designs to this chair (made from flat wood sections), on casters, to make it appear highly likely that it is one of his designs.
http://hwfurniturearchive.bcuc.ac.uk/asset-bank/action/viewAsset?id=1707...
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