Zenith Class watches

The Zenith Class Sport El Primero is a 40 mm steel watch from Zenith's Class line, and it carries the name of the brand's El Primero chronograph movement. An automatic Class El Primero in a 38 mm case had been in Zenith's range since the late 1990s, and a hand-wound Class El Primero HW, with calibre 420, appeared at the Basel fair in 2000. Zenith had become part of the LVMH group in 1999.

The El Primero was unveiled on 10 January 1969 as one of the first automatic chronograph movements. It beats at 36,000 vibrations per hour (5 Hz) rather than the more common 28,800, which lets the chronograph resolve tenths of a second. It survived the quartz years because Charles Vermot, one of the watchmakers who worked on it, hid the equipment needed to make it in the attic of Zenith's Ponts-de-Martel facility when the company turned to quartz. Rolex later used the El Primero in the Daytona from 1988 to 2000.

The Class Sport El Primero references share a 40 mm steel case under sapphire crystal and differ in dial and fitting, which the reference suffix spells out: /04 is the silver dial and /24 the black, M510 the bracelet, and C491 (silver) or C492 (black) the strap. So the 03.0510.400/04.M510 is the silver dial on the bracelet, and the 03.0510.400/24.C492 the black dial on a strap.