@rrrrrrichie. No one in Penzance has a clue; I am following up a lead that I had when I discovered similar marks in Truro, and I am in the process of photographing them, as well. Also spent a day photographing the Cathedral inside and out. I have a copy of Graeme Kirkhams Survey of Truro, currently 'going through'. I intend to write to HIS, in case anyone has trailed this one in the City.
For its date, I wonder if it relates to Gas and Water passing routes. Others clearly relate to owners. It was suggested that it related to 'contractor gang's names. I have been working on the Gas, as this seems most likely, being something worthy of some respect as to a stray pick axe.
