I'm probably throwing more confusion into this but ...
Looking at a map on old-maps.co.uk - the Glan Conwy Corner (Black Cat) area in 1914-1916 had a building known as Pen-Sarn - is it the name of the house opposite Black Cat (which has the boat yard behind it?).
The cottages alongside the railway, and opposite Richard Williams Builders, is known as Pen-sarn Cottages and is also on the map. The bridge over the railway at that point is known as Pensarn Bridge. On the old map the road from Black Cat to Pensarn Bridge is shown as Sarn Mynach.
Llandudno Junction did come under Llangwstenin area at one time, not sure where the boundary was. The eastern section of Llandudno Junction is called the Pensarn Ward in local elections.