I don't know why Arturio Merzario and Ignazio Giunti aren't in this list - they were the only other two drivers for Ferrari while the Heuer sponsorship was in place.
: - Carlos Reutemann (1977 & 1978)
: - Clay Regazzoni (1970 to 1972, then 1974 to 1976)
: - Jacky Ickx (1971 to 1973)
: - Niki Lauda (1976 to 1976)
: - Mario Andretti (1971 & 1972)
: - Gilles Villeneuve (1977 to 1979)
: - Jody Scheckter (1978 & 1979)
So, assuming they did get a watch each (Ignazio Giunti died before the start of his second season, but I believe the watches were given out at the end of the season), that would make 9 Ferrari drivers and 5 ambassadors, i.e. pretty close to the 15 Jack Heuer recalls.