Pattern #19 in the book.

Sizes and back dimensions:
S1: 0-3 months (10.25”)
S2: 6-9 months (11.375”)
S3: 12-18 months (12.375”)
S4: 2 years (14.125”)
S5: 4 years (15.375”)
S6: 6 years (16.5”)

Notions:
Size 1: three buttons
Sizes 2, 3, 4: four buttons
Sizes 5 and 6: five buttons