Joe Barnett stated, Mary Kelly lived at Carmarthen or Caernarvon. The following place names sound very similar, at least to my English ear.
Cwmavon, Torfaen, a small village in Monmouthshire, South Wales
Cwmavon, Neath Port Talbot, a large village in Neath Port Talbot, South Wales
Carnarvon and Caernarvon are older forms of the name of the town in North Wales currently known as Caernarfon.
Carmarthen, South Wales and the largest county.
Barnet didn't know whether Mary Kelly came from South or North Wales. I doubt, therefore, he was aware of the villages named Cwmavon.