Complex sentence: At the time the CAC 40 did not exist yet and the French stock exchange instead published at the end of each trading day a trend indicator of the French stock market computed using the closing value of the 240 largest stocks by market value.
Split into simpler sentences: At the time the CAC 40 did not exist yet, the French stock exchange instead published (at the end of each trading day) a trend indicator of the French stock market. The data was computed using the closing value of the 240 largest stocks by market value.