5963 computer grade dual triode production dates 3505

The first 4ESS was installed in Baltimore, MD in 1964. At the same time, Long Lines was installing underground "hardened" 4Es across the country as part of the AUTOVON military network. That predates SS6 by quite a long time, but surely the switch was retrofitted later. There was no XBAR at all in Baltimore, as it was the Labs' "ESS City" that had been first envisioned in the mid '50s when the ESS project first got off the ground. That machine, although completely retrofitted, is still in service.

You may recall that Baltimore was the first "TouchTone-only" city as well, as ten button 2500 tel sets were the only option. Rotary sets were not deployed there after the last of the 1A-ESS end offices was finished in the early '70s. Also, all "N" and other short haul analog carriers serving the area were wholesale replaced with early DS1 systems, another first, in order to feed the 4E directly without T-CXR-to-analog channel banks at the toll office. The Labs used the Baltimore experience to pattern its nationwide elimination of electromechanical switching and analog short haul carrier that came later. Much of the documentation and procedures of the big LA cutover of the 70T and 92T 4E switches in 1979 and 1982 came from the Baltimore project, as they all did. Both of those mbuttive cutovers happened without a single minute of call blockage in the metro area, a feat that could not be done today. The only impact on callers the next morning was that their toll calls zipped through a lot faster and there was no noise on their toll trunks in the city anymore.