A very similar chair is illustrated in Victor Chinnery, Oak Furniture, The British Tradition, Woodbridge, 1979, p.88, fig.2.73.
Another similar is illustrated as fig. 7 in Dr. R. Ryder Three Legged Turned Chairs Connoisseur, December 1975 Page 243 - 247
A number of closely similar triangular seated chairs have featured in various collections, including the Rous Lench collection, Sotheby's, July 1986, lot 709 (yewtree), The Moller Collection, Sotheby's, November 1993, lots 16 and 17 (both ash), The Dr. Richard Ryder Collection, Sotheby's, July 2002, lot 105 (fruitwood)
There are also a number of yewtree chairs of this model at The Welsh Folk Museum, St. Fagan's, Cardiff