After a bit of research, I found out that the folowing airlines had paid deposits to secure their places in the delivery schedule:
Air France, BOAC, Pan Am, Quantas, Continental, Middle East Airlines, American, TWA, Eastern, United, Air Canada, Braniff, Japan Airlines, Lufthansa, Air India, CAAC and Sabena, plus two aircraft for the Shah of Iran...
Pan Am was the first one to pull out on 31 January 1973 (oil crash) followed by TWA, then the rest, leaving only Air France and BOAC as (obligatory) customers...