Realizing that the Chronomatic announcement was imminent, Zenith rushed to preempt them. The unveiled their "El Primero" ("The First") movement and watches in a quiet press conference in Geneva on January 10 of the same year. They had begun developing their integrated automatic chronograph at the beginning of the decade, with the project stalled and restarted later. By late 1968, a few El Primero prototypes had been produced and these were shown to the world in January. It was not until late in 1969 that Zenith was able to produce customer-ready movements and watches. By this time, both the Chronomatic and Seiko 6139 were already in production and available to customers.
The development of the El Primero had already begun in 1967. While the Calibre 11 "Chronomatic" only made 19,800 semicycles, the El Primero oscillated with 36,000 semicycles, which was unusual for the time. In the 1960s the engineers at Zenith had succeeded in doubling the frequency to 36,000 oscillations.
The first model of the 'El Primero' was available in two versions: Cal. 3019PHC had the unmistakable date display at the 4:00 location while Cal. 3019PHF featured the day, month and moon-phase indication. The weight segment in tungsten carbide registered even the smallest, otherwise not perceptible movements of the wearer's arm. The power reserve amounted to more than 50 hours, the weight segment was embellished by the company logo. The El Primero movement has a diameter of 30 mm and with only 6.5 mm is very flat. Due to its high oscillation rate, it is also the only chronograph movement, which permits a short-term measurement of 1/10 seconds. Other advantages are the instantaneous date change and a dry lubrication on molybdenum-sulfide basis.
In 1985, Zenith resurrected the line, reissuing Cal. 3019PHC as Cal. 40.0. The next year, the movement was renamed Cal. 400. It remains in production today. Because of the unconventional date window placement at 4:30, Zenith created a new calibre, Cal. 400B, which moved the date to the 6:00 position.