According to Cook and Wedderburn (XXI.305), the Supplementary Cabinet was 'a cabinet which Ruskin placed in the School but did not fill'; the examples which they listed were placed there shortly before their catalogue was published in 1906.
Presumably presented by John Ruskin to the Ruskin Drawing School (University of Oxford); first recorded in the Ruskin Drawing School in 1906; transferred from the Ruskin Drawing School to the Ashmolean Museum c.1949
Taylor, Gerald, ‘John Ruskin: A Catalogue of Drawings by John Ruskin in the Ashmolean Museum, Oxford’, 7 fascicles, 1998, Oxford, Ashmolean Museum, no. 314
Ruskin, John, ‘The Ruskin Art Collection at Oxford: Catalogues, Notes and Instructions’, Edward T. Cook and Alexander Wedderburn, eds, The Works of John Ruskin: Library Edition, 39 (London: George Allen, 1903-1912), 21, cat. Supplementary no. 177