Hi A.T. The listings that I know of for the 642 m hill at SH 648 229 are as follows: 1940 Ted Moss 'Diphwys Point W', 1962 William McKnight Docharty 'Tiny W.S.W. Top', 1974 Philip H Cooper 'Diffwys (subsid)', 1989 John and Anne Nuttall 'Diffwys West Top', 1992 David Purchase 'Diffwys W Top', 1992 Terry Farrar 'Diffwys B', 1993 Harveys 'Diffwys West Top', 1995 Michael Dewey 'Craig Bodlyn', 1997 Robert Jones 'Diffwys-West Top', 1997 Alan Dawson 'Diffwys West Top' and 1999 Dewi Jones 'Diffwys West Top'. As you can see a name incorporated around 'Diffwys West Top' is the one that has most been used, the hill itself remains seemingly unnamed on any Ordnance Survey map.
My enquiries with farmers in the upper Hirgwm (SH 659 208) were some of the first I made. Robert Jones has lived in the upper part of this high valley all of his life (aged 66 in December 2006) and worked on this land since the age of 15, whilst Arfon and Wyn Owain farm from close to the minor road end that makes its way up from Bontddu.
Arfon led me through every bounded ffridd, with every enclosed field having its own name. The bounded land directly south of the 750 m summit of Diffwys is known as 'Mynydd Hafod', this is the land that has 'Craig Aderyn' on it, this land is part of the cynefin of the farm 'Hafod-uchaf' which is situated at SH 659 210. Directly to the west of this land is firstly ' Mynydd Ffridd Du' and secondly 'Mynydd Cae Goronwy', the respective farms associated with each cynefin can be found in Hirgwm. I was told that where the land of 'Mynydd Ffridd Du' and 'Mynydd Cae Goronwy' meet is open land and very close to this 642 m high summit, but the highest point is just on the land known as 'Mynydd Cae Goronwy'. A later conversation with Robert Jones confirmed what Arfon Owain had told me. It could be deemed appropriate for this hill to take on its name from the named land that its summit is situated on. The name of 'Mynydd Cae Goronwy' could be translated as 'Mountain at the head of Goronwy's Field'. The more you look at O.S. maps the more hill names associated with near farms you find. There's gazzillions of them!!!