The effects of Vatican II, which ended in 1965, didn't take until 1976 to manifest themselves.
I may not undertand your reply. But many of the changes in the Church that began with the 2nd Vatican Council that were so disconcerting to more traditional Catholics took years to go into effect. They then had to grapple with those changes over a period of time. People don’t just walk away from their birth religion overnight. So I can easily see how a family that could no longer identify with the Church as they knew it would take till ‘76 to make a final decision to leave and to find another church.