The coupon itself first appears on baseball cards in 1927, hand stamped on an occasional card. Here are examples from 1927:
It went full-printed in 1928 on various series of cards. By 1929 it was all over the place, as on this 1929 copyright office file copy of a 1929 Tenderfoot card:
Here is a page from a 1930 Exhibit catalog explaining their coupon plan.
