|credit: Ashmolean Museum, Oxford|
In his preface there is a section in which he addresses Welsh readers: "AT Y KYMRY". It was translated into English by William Nic(h)olson, Bishop of Derry, and published as an appendix (pp. 216ff.) to his book The Irish Historical Library in 1724. On page 219, which you can see here, one finds the term "receiv'd Pronunciation".
Where is the avid reader to find a yet earlier record of the noun phrase?