It should not be forgotten that a significant contributor to the resurrection of the El Primero in 1984 was the mighty Rolex Watch Company, which had never jumped on the quartz bandwagon. At this time the Rolex Daytona Cosmograph, a manual-wind chronograph, was powered by the iconic Valjoux 72. Rolex wanted to go automatic ("perpetual" like its other watches), and so sniffed around Zenith, who'd produced the best automatic movement in the El Primero. They closed the deal, and so Zenith supplied its movements to Rolex for the Daytona from 1985-2000, greatly assisting Zenith in resuming its own production.
Rolex, of course, had to dumb the movements down from 10 to 8 beats per second (so they could service them), and eliminated the date (Rolex design tolerated a date window only at 3 o'clock, which wasn't feasible due to the minute register). So, for about triple the price, you could get an automatic chronograph without a date and slowed down to mid-century standards. Fools and their money ....
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