Carreg Fedyddiol, The Stone of Baptism, is a cross shaft, with a socket in the upper surface for a cross head, now lost. It received its name as it was previously thought to have been a font bowl.
It originally stood by the spring at Cae Fynnon, but now forms part of the Margam Stones Collection, housed in the old school house, Margam (NPRN 94512).

Source: Knight, Jeremy K. 1999. The Margam Stones: Cadw Guide

K Steele, RCAHMW, 10 November 2008