1)
I use the classifications of packaging styles as defined by the Airfix Collectors Club, which are explained in detail in the highly recommended website Collecting Airfix Kits .
Three packaging styles are relevant here : Type 1, Type 2, and Type 3 . Type 2 headers resp. boxtops have been used by Airfix since about 1959 / 60, Type 3 headers and boxtops has been used from 1963 to 1973.