William Jones FRS (1675-1749), a friend of Sir Isaac Newton and Sir Edmund Halley, was the pioneering mathematician who innovated using the Greek letter 𝛑 to refer to the ratio between the circumference and diameter of a circle. (Spelling it out like that shows you why he thought the symbol would catch on!)
Jones was born not far away from the Ysgol Gynradd Ty Mawr primary school which stands across the road from the Maenaddwyn menhir. Sadly the school closed down in 2010 and now appears to be a private residence, but the old school sign and the blue plaque commemorating William Jones remain.
I've no idea why the menhir was erected – whether it had religious or political significance, or both – but it's quite likely that Jones knew the stone if the road through Capel Coch has been in the same place for a few hundred years.
NB: this stone should be classified 'Access: 5' as the stone is both drivable to and accessible by the disabled. It's directly adjacent to the road, and as an earlier post on this page says, there is even parking!
There is a photo of the school on Wikimedia Commons.
Something is not right. This message is just to keep things from messing up down the road