The Ferris Loop bridge
The thing the tradition is named after was a piece of test equipment, and the people who built it would not have understood the question.
Cordell Telephone & Signal ran an independent plant across a long, thin service area with four unstaffed repeater huts on the Ferris route. In 1971 the company cut a conference bridge into the maintenance pairs so that a craftsman at one hut could raise the others without going through the operator at Marchford. It was six drops and a resistive summing network in a rack at the Ferris hut. There is a drawing.
That is the entire technical content of the origin, and it matters mainly for what it is not. It was not clandestine, not clever, and not anybody's idea of a network. It was a way to avoid a phone call.
How a test facility became a place
The bridge had two properties nobody designed for.
It was always up. Maintenance pairs are not billed and not torn down. A craftsman who raised the bridge and said nothing was not costing anyone anything, and there was no reason for the company to notice.
It had no operator. Everything else on that plant went through Marchford, where someone knew who you were and wrote it in a book. The bridge did not, and so the bridge was the only circuit on the route where a conversation left no record of having occurred.
The second property is the one the later tradition takes as doctrine and the first is the one that actually did the work. People sat on it because it was free and because it was there, and the manners that grew up on it — do not ask who is on, do not log, say what the plant is doing and not what you are doing — were the manners of men who were technically not supposed to be chatting and had decided, without discussing it, not to make that anyone's problem.
Every convention in lore/lexicon descends from that: an etiquette invented to avoid an argument nobody wanted to have with a foreman.
What is actually attested
The drawing, in a Cordell plant binder, showing six drops and dated March 1971. It is the best-evidenced object in this folder and the least interesting.
Everything about use is secondhand. There are no recordings, no logs by construction, and the men on it are named nowhere, because the lore/marchford ledger recorded circuits and not conversations — which is the convention the Loop later adopted as a principle and which began as an accounting practice.
The bridge is generally said to have been removed with the route in 1983. Nobody has produced the work order and the date is inferred from the huts going unstaffed, which is a different event.
See lore/the-plant for what the equipment could and could not do, and lore/quiet-exchange for the eleven minutes in 1978 that the whole tradition is built on and which the bridge itself cannot corroborate.