In 1988, after the discontinued production of the manual winding Daytona, Rolex launches the first references featuring a self-winding chronograph movement and sapphire crystal: ref. 16520 in steel, ref. 16523 steel-gold and ref. 16528 in yellow gold.
All three references are housing the Zenith El Primero based caliber 4030. Historically it is quite impressive that Rolex indeed did not have an automatic in-house movement for Chronographs until the introduction of the reference 116520 in 2000. In fact, they had a ten-year contract with Zenith in order to guarantee the supply of this key component.
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