Hi Anne,
Matthew Grant, as I recall, was the record keeper for Windsor, Connecticut. I found this out when I signed up to become a descendant of ancient Windsor. He is a 10th GGF along with several other early Windsor settlers. I expect his keeping his own records led to his keeping records for the community.
If he listed his mother and her death date, I agree with Gene that this would be first-hand information! He should know his own mother. Hence, she should be connected and be PGM.
Thanks, Anne!