I know that one revision was to allow it to provide music on hold prior to the introduction of the ComKey system around 1975. I'm thinking that the G version came along shortly thereafter to make them more easily-identifiable with the pretty blue handle. The H Version (as I was told by an old-timer from C&P Telephone) was to address the issues with power consumption, relay contact wear and noise. Apparently, customers were complaining about the clicking. I'm not so sure that I believe the noise part since every manufacturer was going solid-state in the early 80s.

The 400G also introduced modular option plugs, rather than punch-down option straps. It was said that after repeated refurbishing of previous 400 versions, the solder joints at the bases of the punch down clips became vulnerable and prone to phantom issues.

Let's also remember that there was a drive by many businesses to simplify everything at the time, and the Bell System was all about efficiency. Their stuff was rented, so making it last longer between service visits meant increased revenue. That makes perfect sense to me. With the 400G, you had only a few choices on how to place the jumper plugs. You really couldn't mess it up. Bell was big on goof-proofing their hardware.

The in-use LED was also handy in large installations. We used to flash a line repeatedly while someone would listen for the clicking card. What a pain. It was much improved when this flashing could be seen in a large rack. I swear that ITT and others didn't come out with this feature (as in ITT's 400E) until long after WE came out with their 400G, as in ten years.

Ed Vaughn, MBSWWYPBX