Lap belts was starting to get optional in most american cars by then, but probably not so heavily advertised since Ford made a big deal of them in 1956, which got the opposite reaction by the public, people stayed away from Fords, thinking a car where you needed seatbelts was too dangerous… 
1959 was also the year when Volvo started to put 3 point belts as a standard feature in their cars, which was pioneered by them.