I lived in Montreal for the first 38 years of my life. I had never heard of butter tarts until we moved to Toronto 21 years ago. My family was really into food when I was growing up and would likely have known about butter tarts if they were available, although we were not fans of tarte au sucre, which is similar in some ways.
Someone I went to school with says she remembers them being sold at Woolworth's, Steinberg's, and Cantor's Bakery. I think she's mistaken. Does anyone of my "vintage" or a little older or younger remember butter tarts being available in Montreal when you were growing up?