Just to build on some previous posts, here are my current thoughts.
At Bedd yr Afanc there appear to be at least 16 stones incorporated in the monument -- it all depends how many of the "outliers" you choose to incorporate.
The stones are in two rough rows, and only project above the surface for 40 cms or so. They vary a lot in shape and dimensions -- and rock type too. Some are made of dolerite, some are volcanic ashes, rhyolites and what appear to be gabbros. The stones have almost certainly just been picked up in the immediate vicinity and built into the monument. They are typical glacial erratics made of rocks from the Fishguard Volcanic Series and from related dolerite and other intrusions -- and as far as I can see not one of them has travelled very far from its place of origin. Not one of them can properly be described as a "pillar". The facets and broken surfaces seem to be of several different ages, but none of the edges is particularly sharp.
Where are the capstones? There are plenty of flattish stones embedded in the turf in the vicinity. Some may actually be embedded in the turf in the "passage" itself.
Many of the stones lean inwards towards the axis of the monument. The passage is not aligned towards anything significant....... which seems to upset some people.
There are a number of boulders with rounded-off edges in the vicinity which seem to be made of a very similar bluish foliated rhyolite as that which is exposed at Rhosyfelin. The rock outcrops there are less than 2 km away, towards the NNE. Without proper geological analysis, the origins of these boulders cannot be reliably guessed at -- and there are of course many other outcrops of bluish foliated rhyolite in the Fishguard Volcanic Series to the N and NW.
If (as I suspect) the passage grave was made of stones picked up in the immediate neighbourhood, one might expect some of the "casts" of these stones to be revealed in any excavations. I hope that the archaeologists take note of that point. I can see it now -- they all start digging, find some casts, call them sockets, and rush to the conclusion that they have found "proto-Stonehenge" and that lots of monoliths were carted away from here to be re-set into the ground in the bluestone circle on Salisbury Plain. If the casts are scattered about, as I would expect, bang goes the proto-Stonehenge theory. If they are demonstrably arranged in a circle or oval, then we start to get into interesting territory! Let's see the colour of the evidence.
My tentative suggestion at the moment is that the geology at Bedd yr Afanc is dominated by thick glacial deposits that have been carried into the area by Irish Sea Ice travelling broadly from NE towards SW. That means that stones from the Craig Rhosyfelin outcrops might have been carried to the vicinity of Bedd yr Afanc and then dumped there. Other foliated rhyolite boulders may not be erratics at all, but may be locally derived. I hope that some serious geology can be done here, to work out the approximate provenances of the 16 or so stones used in the passage grave.