Says McLuhan Biographer, W. Terrence Gordon:
"by the time it appeared in 1967, McLuhan no doubt recognized that his original saying had become a cliché and welcomed the opportunity to throw it back on the compost heap of language to recycle and revitalize it. But the new title is more than McLuhan indulging his insatiable taste for puns, more than a clever fusion of self-mockery and self-rescue — the subtitle is 'An Inventory of Effects,' underscoring the lesson compressed into the original saying."
Freudian slip? :)