The photograph above shows a fragment of carved stone discovered in the offices of Elderslie Estates by John Newlands. Its inscription matches a stone mentioned in the Old Statistical Account of 1845 as having been dug up from a garden adjoining an old tenement, said in the mid 19th century to occupy the site of the birthplace of William Wallace. The NSA account indicates that the stone was subsequently taken to Elderlie House, an event commemorated on the plaque below, along with its subsequent relocation to Houston in 1911. How or when the stone came to be in the estate office is not known. Further information on the site where the stone was found is available here.
Plaque, image copyright John Newlands.