The stones that I found on the CARN site are B/A with a question mark. Dyffryn seems to be in South Wales. Puncheston is interesting(6 km east of Letterston) because it lies at the foot of Mynydd Castlebythe with tumuli on top., there is another, Mynydd Cilciffeth to the N/W, with cairns on the top, this is map work. I am not going to use the CARN site if it is in a mess as Kammer says. My interest in Garn turne is that it maybe a focal point, their is a rocky outcrop by the site, and I just wondered if this was the reason Garne turn was erected, if it could be a primary sanctuary site, a bit like The Mothers Jam down in Avebury, it has a "V" shaped courtyard like that of Pentre Ifan.
Moss

Searching CARN Puncheston Stone is in fact Carreg Quoitan , and St.Brynach's Stone is more usually called St.Brynach's Cross because they stuck a cross on top . So those two grid references check out . Both SN coincidentally .
But the listing you would have seen on CARN is a pigging mess . On another thread was mentioned the confusion of Parc Carreg and its alternates all over Wales . Here they popped up again , and other multiples too . Some look like variants for the same site and some very slight differences in grid references seem down to the different databases used . But trying to track things down it does look a little strange that places on RCAHMW at the very late SM numbers often have 'duplicates' at very early SN numbers on the likes of TMA and TMP and Megalithia !
My favourite site on the list is the schizophrenic "Homer Longstone ; Longstone ; East Longstone ; West Longstone" !!
Here are a few non-RCAHMW alternates :
TMP Parc y Mean Llwyd SN164397 , Parc Maen Stone SN113283 , The Longstone SN146094
TMA Parc y Garreg SN152114 , Parc Gerrig Hirion SN008388 ( Dinas crossstone ) ,
Mega Longstone SM893073 , Carreg Wen SN399340 .
Cerrig Meibion Arthur SN118310 may also be a Ty-Newydd it seems .