The "bluestone that got away" -- this pillar in the centre of the "bluestone quarry" at Carn Meini is often assumed to be a pillar that was intended for Stonehenge, but was for some reason left behind. Less romantically, we might think of it as a designated gatepost which Mr Jones of Mynachlogddu planned to take down to his farm in 1850, but never quite got round to it.
Maybe I have been a bit unkind to HH Thomas -- on looking back over the literature I'm still pretty convinced that he "aggregated and simplified" his geological samples so as to "demonstrate" that they all came from the eastern end of the Preseli Hills -- but to his credit he did at least realize that they were variable enough to have come from a number of different sources in the area, including outcrops of spotted and unspotted dolerite, rhyolite and volcanic ash. He did believe in the long-distance human transport theory (indeed he was the original proposer of it) but he thought that the bluestones were aggregated together in a sort of "proto-Stonehenge" in the parish of Cilymaenllwyd, and that that monument was later dismantled and transported off to Stonehenge for some "ritual" reason. He thought that the bluestones might have been short-travelled glacial erratics, carried downhill from their sources in the mountains by an upland glacier, and assembled together in one neighbourhood by some freaks of glacial transport.
So the bluestone quarry came later. The man whom we can blame for that was Richard Atkinson, who became the leading Stonehenge archaeologist and propagandist in the years after the Second World War. His book on Stonehenge, published in 1956 and reprinted many times since, became the source of all wisdom..... and in it he emphasised that the bluestones, of various kinds, had come from a very restricted area around Carn Meini, and that they had been CHOSEN by tribesmen who were very determined and who had the great technical skills needed to transport the stones over very rough terrain and across the sea. In Chapter 2 of the book he emphasised that the bluestones at Stonehenge are almost all in their natural state, matching very closely in size, shape and appearance the "columnar and slab-like" boulders to be found today on the jagged outcrops of Carn Meini. I don't think he used the word "quarry", but it was inevitable that others would use it after reading his text and having encountered his very forcefully expressed ideas.
Atkinson (referred to sometimes nowadays as "that old fraud") was a very powerful and forceful character, and in the decades after 1956 his ideas about Stonehenge became "the orthodoxy." Although he originally flagged up the diversity of the bluestones, it suited him well to concentrate on Carn Meini and the "very restricted area" from which the majority of the stones had come. And of course, if the worthy neolithic tribesmen were capable of extraordinary engineering skills in the stone transport department, it would have been no trouble at all for them to do a little simple quarrying at the source. So in 1956 Atkinson paved the way for Profs Wainwright and Darvill, more than half a century later........