I remember that Phyllis Newman did all the matinees for the show - instead of Barbara Harris. It was quite a new idea that actors didn't do all 8 shows at all times.
Matinee alternates actually weren't uncommon in the 1960s -- and they were certainly more common then than now.
Harris did all performances for the first two months of the role, with Newman beginning in December 1966. As the run progressed, Harris began to miss a significant amount of performances and then eventually left rather abruptly.
"You travel alone because other people are only there to remind you how much that hook hurts that we all bit down on. Wait for that one day we can bite free and get back out there in space where we belong, sail back over water, over skies, into space, the hook finally out of our mouths and we wander back out there in space spawning to other planets never to return hurrah to earth and we'll look back and can't even see these lives here anymore. Only the taste of blood to remind us we ever existed. The earth is small. We're gone. We're dead. We're safe."
-John Guare, Landscape of the Body