In the intervening decades, the El Primero found its way into an increasing variety of Zenith watches, and at the same time, it underwent technical evolution as well. Two of the most important advances upped the resolution of the chronograph. A 36,000 vph chronograph was a major technical achievement in 1969, and in 2010, the next step was taken with the El Primero Striking 10th. While the Striking 10th has the same frequency as the original El Primero, it has a new chronograph gear train that supports  a central hand that makes one revolution every 10 seconds, with a 30-minute counter at six o’clock and a 60 second counter at three o’clock – allowing for direct read-off of 1/10 of a second intervals. And in 2017, the Zenith Defy 21 was introduced, featuring two separate balances, one vibrating at 36,000 vph, and the other at 360,000 vph, allowing it to time intervals with a 1/100th of a second resolution. The original El Primero ref. A386 of 1969 has been exactly reproduced for the 50th Anniversary Collection trilogy, and the Striking 10th is present in the Chronomaster 2 El Primero, with the Zenith El Primero Defy 21 rounding things out.