Anchovy toast gives up the second slice on purpose. Where the closed paste sandwich seals a thin film of fish between soft bread for a lunch tin, this is an open-faced thing made and eaten at once: anchovies mashed with butter and spread on thin, hot, properly crisp toast, in the Gentleman's Relish manner where a tiny amount of an intense savoury spread sits on a single firm base. There is no top slice to balance the salt, so the toast itself has to do that work. The defining fact is the bread state, not the fish: a soft slice would collapse and read as a smear, and the whole effect depends on a brittle, dry base under a strong topping.
The craft is heat, timing, and proportion. The toast is taken further than it would be for a sandwich, firm and dry to the centre, because it has to carry oily mashed fish and warm butter without going soft before the last bite. The anchovies are pounded smooth with butter so they spread rather than sit in lumps, and the layer is kept deliberately thin, since the format has no counterweight and the contrast it relies on is a sharp savoury hit against plain crisp bread. It is served immediately and eaten over a plate or with the fingers, often cut small for a tea tray, because a thin slice of hot anchovy toast does not survive the wait that a closed sandwich is built to survive. This is its strength and its constraint at once.
The variants stay inside the open-faced savoury idea. Sardines or smoked mackerel mashed onto toast trade one oily fish for another; the yeast and beef spreads run the same single-slice logic with a darker note; a scrape of the proprietary potted relish is the most concentrated reading of all. Each is its own form and deserves its own article rather than being crowded in here.