Dead Letter
Ossuary's board. Opened 1986, four months after or four months before lore/nailhouse depending on whom you ask — see lore/the-1986-problem, which is a better argument than it sounds.
One line, for its entire life, deliberately.
The shape of it
Where Nailhouse was small and quiet, Dead Letter was small and extremely loud. The message base ran to something over eleven thousand messages, the great majority written by the operator. Ossuary posted daily, at length, and about everything: the plant, the Count, the tone tables, the weather, whether a convention was a convention or just a habit that had gone unchallenged.
The single line was a position. Ossuary's stated view was that a board with two lines is a place where people talk to each other and a board with one line is a place where people talk to the board, and that the second is more honest about what a message base actually is.
The name is a postal term for undeliverable mail held at a central office. He did explain this one, more than once.
The Count, and the argument
Ossuary held that lore/long-count admitted exceptions — that a sit interrupted by plant failure the sitter did not cause was still a sit, because the Count measures intention and endurance and not luck.
He argued this for eleven years, in public, at length, against a man who never answered. The one-sidedness is not incidental: see lore/halberd-and-ossuary. Reading Dead Letter's message base straight through gives the strong and false impression of a feud, because only one side of it was ever written down, and lore/nailhouse kept a roster instead of a position.
The concession
The lore/armistice-banner, 12 July 1997. Fifteen lines, naming nobody.
What it gives up is precisely bounded and worth being clear about: Ossuary does not abandon the view that the Count admits exceptions. He abandons the view that his exceptions and Halberd's are different rites. Since the operational problem was two rosters and one caller, that concession dissolves it without either man conceding the point of doctrine.
It is generally treated as the most graceful thing anyone in this tradition ever did, and the ten-month gap between the lore/marrow-tape hand-off and the banner is why most people think the two are connected.
After
Dead Letter is the only board here with no established end date. It was still answering in 2001. It was not answering by some point in the mid-2000s, and the several people who called in between report a board that was up, current, and being posted to by nobody at all.
That is not the lore/vesper-node situation and should not be confused with it. Vesper answered for six years after its operator stopped. Dead Letter simply went quiet in the ordinary way while remaining switched on, which is a much duller thing and has attracted a great deal more speculation than it deserves.