Like the Patek Philippe Calatrava ref. 6006G featured yesterday, the Zenith Chronomaster El Primero Full Open was another under the radar introduction at Baselworld 2017.

Not in the Zenith press kit or website, the El Primero Full Open is a skeleton dial version of the retro El Primero. And it’s available in both the largish 42mm case, as well as a 38mm size that feels almost like a vintage watch, being the same size as the original El Primero A386 of 1969.

The 42mm

The El Primero Full Open has no dial, exposing all the mechanics usually hidden. This reveals bits of the chronograph mechanism, as well as the open-worked date wheel. While not new – the open dial movement was also used for the El Primero Lightweight – it’s the first time this look is found on a relatively affordable Zenith chronograph.

Detail of 42mm

Notably, the movements inside the 42mm and 38mm versions are slightly different. The larger watch has a slightly larger date wheel to maintain the proportions. Notably, the skeleton date wheel on the 38mm has a more refined look, having thinner numerals.

Note the large date disc on the 42mm

The smaller case size, known as the El Primero Original, is more visually interesting because of the smaller dial, which gives it a more mechanically dense appearance. It is, however, somewhat small by modern standards, and let down by an oversized Zenith logo that’s as large as it is on the 42mm model.

The 38mm

With a smaller date disc that has slimmer numerals

But the 38mm is appealing because it’s an unusual combination: the open dial is reminiscent of modern-ish sports watches from the likes of TAG Heuer, while the case size and style feel like a vintage reissue.

Most of the differences between the two are cosmetic. Both versions are equipped with the El Primero 400, the self-winding chronograph movement that runs at 36,000bph that’s been in production since 1969. Most of the movement has a dark grey galvanic finish with straight graining, giving it a workmanlike appearance.

The 42mm from the back

And the 38mm model

Both the 38mm and 42mm models are available in stainless steel or steel with an 18k pink gold bezel (fitted with matching gilt hour markers and hands). An option of a steel bracelet is also offered. The individual model references are as follows:

38mm, stainless steel – ref. 03.2153.400
38mm, steel with 18k gold bezel – ref. 51.2153.400
42mm, stainless steel – ref. 03.2081.400
42mm, steel with 18k gold bezel – ref. 51.2081.400
Price and availability 

The steel models are priced at SFr9900 or S$15,000, while the version with gold bezels are SFr10,900 or S$16,500. They will be available in stores starting June 2017.