The first chronograph wristwatch to feature an opening on the dial to exhibit the beating heart of the movement, the Chronomaster El Primero Open has become something of a modern icon among chronographs.

Zenith was the first Manufacture to partially reveal the beating heart of the movement through an opening on the dial. The El Primero EP4061 is an upgraded version of the original El Primero automatic high-frequency chronograph calibre, fitted with a silicon escape wheel and pallet lever that require no lubrication.