In 1658, William Maurice made a catalogue of the most important manuscripts in the library of Robert Vaughan of Hengwrt, in which 158 items were listed.1 Many copies of Maurice’s catalogue exist, deriving from two variant versions, best represented respectively by the copies in Aberystwyth, National Library of Wales [= NLW], Wynnstay 10, written by Maurice’s amanuenses in 1671 and annotated by Maurice himself, and in NLW Peniarth 119, written by Edward Lhwyd and his collaborators around 1700.2 In 1843, Aneirin Owen created a list of those manuscripts in Maurice’s catalogue which he was able to find still present in the Hengwrt (later Peniarth) collection.3 W. W. E. Wynne later responded by publishing a list, based on Maurice’s catalogue, of the manuscripts which Owen believed to be missing, some of which Wynne was able to identify as extant.4 Among the manuscripts remaining unidentified was item 33, the manuscript which Edward Lhwyd had called the ‘Hanesyn Hên’.5 The contents list provided by Maurice in his catalogue shows that this manuscript was of considerable interest.6 The entries for Hengwrt 33 in both Wynnstay 10 and Peniarth 119 are identical in all significant respects. These lists are supplemented by a briefer list compiled by Lhwyd and included elsewhere in Peniarth 119 as part of a document entitled ‘A Catalogue of some MSS. in Hengwrt study Ao 1696’.7 In table 1 below, the latter’s entry on Hengwrt 33 is printed in parallel with the text from Wynnstay 10. The contents list of Hengwrt 33 published in Lhwyd’s Archaeologia Britannica is evidently a conflation of Maurice’s list and the list in Lhwyd’s 1696 catalogue, a circumstance which led to the accidental inclusion of the vernacular chronicle O Oes Gwrtheyrn at two different points.8