I'd looked into this in detail a little bit ago. The change happened in May, Jerry's car is one of the first. April cars are all top posts. I had it narrowed down to exactly when, but I need to look again.
Usages:
67, 68, and early 69 L6, 307, and Z28: The standard battery was the Delco Y55 (reverse polarity)
67, 68, and early 69 327, 350, 396, and 427: The standard battery was the R59 (61 A-hr).
Late 69 L6, 307, and Z28: The standard battery became the Delco Y77 with side posts.
Late 69 350, 396, and 427: The standard battery became the Delco R79 with side posts.