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In order to hasten the development of electronic switching in the Bell System, the first two No. 1 Electronic Switching Systems (No. 1 ESS) were placed into service without benefit of a field trial. Based upon experience gained in these early offices, hardware and program modifications were made prior to placing subsequent offices into service. This paper describes the program, hardware, and translation changes that were retrofitted into the first two No. 1 ESS offices in order to bring them up-to-date. The retrofit of these changes were successfully made on an in-service basis.