The re-defined boundaries of Penzance were discussed and proposals were made in a report of 1928 (of which I have a copy), eventually in 1934, stones were set.
This one is the Trenow Stone. Others are at Mousehole, Sheffield, Tredavoe, Tregavarah, Heamoor and in Posses Lane (near the airport).
Knowing where they are does not mean that they are easy to find.