The Zenith El Primero was the first high frequency (36,000 vibrations per hour vs 19,800 vph of the Chronomatic and 21,600 vph of the Seiko 6139), fully integrated, automatic chronograph, the first with a running seconds hand and it was only 6.5 mm thick, even slimmer than traditional chronographs!
Zenith and Movado (at the time the two brands were owned by the same holding) explained these features in their announcement: “The Zenith and Movado watch companies have achieved an extraordinary feat in combining two precision watches in one. It comprises a high-frequency automatic watch with calendar along with a timer-chronograph allowing time measurements to the tenth of a second. It is equipped with an hour and minute timer. This is the first watch of its type in the world. The extraordinary feature is that both these mechanisms fit in a space smaller than that of a traditional chronograph. This model has all the advantages of a standard watch plus the date, automatic winding and the chronograph with timers and calendar.”
The first El Primero timepiece that appeared in a Zenith advertisement (March 1969) was the A384 reference.