In the early years, sometimes a gravestone would break for some reason. The ones that had the beloved person would replace the stone with another one and use the broken stone in their home for stairs or other reasons in the home.
There could be a replacement stone in the local cemetery in Sudbury. Hearing the age of the stone, the replacement may not be visible years later. The stone that has been kept in a home for years would not show as much or any signs of deterioration.
I am surprised that the local historical society isn't helping you out with this. Perhaps they do not want the responsibility of the stone as many old homes had stones like this.