This display model was sold on EBay some months back:
Without a display stand it’s difficult to determine exactly who made this, but all indications are that it was an “official” model, made by Boeing, Lockheed or NASA. The design was given some small amount of study around 1973, though the available documentation on it is lean.
Lockheed studied the same idea with the C-5 Galaxy. Of course the C-5 would have been easier to modify since it already had shoulder-mounted wings.