McLuhan seems to have first broached the notion of body percept in a letter to Peter Drucker in October 1966:

Whenever we make a new technology, that creates a new environment which we automatically assume as our cultural mask. We do this via our senses, not our concepts. Each new environment creates a new body percept, new outlook and new inlook (…) The orientalizing of our world by inner involvement in depth occurs via circuit or feedback along with speed-up of data. An all-at-once world is structurally like the subconscious. It tends to be mythical and archetypal. Consciousness becomes incidental rather than structural. It is the old environment, not the new one. The individual yields to the tribal man. Electronic man is the first since neolithic times to live in a man-made environment. Preliterate man naturally regarded his world as man-made. An information environment like ours is man-made. Media are, as it were, cultural or corporate masks. (McLuhan to Peter Drucker, October 24, 1966, Letters 338) 

Over the next two years he recurred to the notion with some frequency:

Each of us forms a body percept, from moment to moment, based upon his intake of sensations, perceptions, but we are completely unaware of this body percept which we form of ourselves from moment to moment. It takes considerable dexterity and skill to observe one’s own body percept, the image we form of ourselves. (Contribution to Technology and World Trade, Session — Technology: Its Influence on the Character
Of World Trade and Investment, November 16, 1966)

Everybody responds to a new environment without benefit of concepts. The immediate sensory adjustments which we make to each and every change in our surroundings also (…) alter our body-percept or our sense of ourselves. (The Future of Morality: inner vs outer quest, 1967)

a feeling of themselves — an image of themselves — body percept (McLuhan in conversation with Norman Mailer and Malcolm Muggeridge in 1968)

the vanishing point in the viewer (…) proprioceptive tension and body percept (Through the Vanishing Point, 59)