The bell
One ring. Not answered. That is the whole convention and it is the oldest thing here that anybody still does.
On the plant
A craftsman raising the bridge changed the level for everyone on it — see lore/the-plant — so arriving was audible whether you spoke or not. The bell formalised what that already meant: one ring on the maintenance pair, then nothing, meaning a drop is live, it is me, I want nothing.
The point was to make presence unremarkable. Without it, an arrival that stayed silent was a question in the room. With it, silence was declared in advance and the room could carry on.
This is the mechanical root of graze in lore/lexicon — reading without connecting to anything, which the tradition explicitly does not disapprove of. Outsiders find that odd because they expect a closed group to resent lurkers. The Loop's equipment made lurking visible and its manners made it fine, in that order.
On the boards
It carried over almost unchanged. Both lore/nailhouse and lore/dead-letter had the convention that a login with no activity and a clean disconnect required no explanation and got none, and that asking somebody what they had been doing on was a breach.
Ossuary wrote about it more than once, characteristically, and the best line is his: a board with nobody reading it is off. The people who read and say nothing are the board being on.
The failed extensions
Two ring, three ring, and a coded interval were all proposed at various points and none of them took.
The reason given at the time, and it has held up, is that the bell works because it carries no information. A signal that means only "present, nothing wanted" cannot be misread, cannot be answered wrongly, and cannot become a thing you are judged on. Adding a second meaning would have made it a message, and a message is a thing you can get wrong.
This is the same argument, arrived at independently and twenty years earlier, that lore/pell makes about the lore/cold-handshake: the moment a practice can be performed correctly or incorrectly, it becomes a way of being someone. The Loop kept exactly one signal free of that and defended it by refusing every enrichment.
Compare the hum on lore/port-4919 — a signal reliably present and carrying nothing — which the lexicon treats as the same idea in a different medium.
Still in use
It is the one convention with continuous practice. People who have never seen lore/fallow-ledger and could not tell you what the lore/long-count is will still, on a channel with Loop ancestry, announce themselves once and say nothing further, and will not be asked.
A tradition mostly loses its doctrines and keeps its manners. This is the clearest case of it in the folder.