Well, the 420s were produced from 1969 onwards. Initially, six batches were built, the last of which was completed in 1981. Various network extensions but required another two batches to be procured. These were treated to various technical improvements, such as the plug doors which replaced the sliding doors present on the first six batches. The 7th and 8th batch sets all date from between 1989 and 1997, and are still in rather good condition. They were also fitted with LED destination signs and a Railvox passenger information system after having been allocated to Stuttgart.
At various points, 420s were also used in the Ruhr area, so for you to have seen them around Essen was not an uncommon occurrence.