Here's another thing that makes the El Primero special: despite the fact that it was produced by Zenith alone against fierce competition by a consortium (Breitling/Hamilton-Buren/Heuer-Leonidas/Dubois-Depraz) to get the world's first automatic chronograph on the scene, it was also designed to have a date function. This might seem normal to us nowadays, but in 1969, it was anything but the rule. There were few chronographs of any kind with a date window before that (some examples by Landeron and Hanhart that I know of) - most had nothing and some Valjoux ones had a central pointer date, which interferes a little with the chronograph second hand legibility. So, not quite a first for the EP but it seems to have been a little bit of a trend-setter in that regard - after it came on the market, practically all chronograph movements were designed with it!

Hartmut Richter