When was prog labeled?

According to Edward Macan: Rocking the Classics. English Progressive Rock and the Counterculture. Oxford University Press 1997, page 26, 27:

"A word should be said at this point about the term "progressive rock" itself. In the mid- to late 1960's, this term was appropriated by the underground radio stations and applied to psychedelic music in general; the label was used to distinguish music of this type from the pop music of the pre-psychedelic era. Around 1970, however, the term "progressive rock" came to have a more specific meaning, signifying a style that sought to expand the boundaries of rock on both a stylistic basis (via the use of longer and more involved structural formats) and on a conceptual basis (via the treatment of epic subject matter), mainly through the appropriation of elements associated with classical music. It is this new, more specific application of the term which is clearly intended in the liner notes of Caravan's debut LP of 1969: "Caravan belong to a new breed of progressive rock groups - freeing themselves from the restricting conventions of pop music by using unusual time signatures and sophisticated harmonies. Their arrangements involve variations of tempo and dynamics of almost symphonic complexity.""