The crab paste sandwich is the made sandwich rather than the jar it comes from, and the distinction matters because everything about it is decided by the scrape, not the filling. Crab paste, a cooked and pounded shellfish spread concentrated to survive months in a sealed jar, is spread thin across soft buttered bread, and that is the sandwich. The inverted ratio that runs the whole paste shelf runs here too: the paste is the thing you use least of, because a paste built for shelf-stability is so salty and so strong that a thin smear is already a full mouthful and a thick layer is inedible. It is the economical seafood sandwich, crab between bread without fresh crab.
The craft is the scrape and the butter under it. Butter is not optional lubrication; it is the carrier that drags the paste's salt evenly across the slice and stops a thin smear reading as a smear rather than a flavour, and a sandwich made without it tastes of a sharp stripe and bare bread on either side. The paste keeps the sweet, faintly briny note of crab but has no flake left in it, so it brings flavour and no texture, which is why the bread is always soft and plain: an assertive crust would have nothing to push against and would only fight a filling with nothing to give back. It is assembled cold, cut thin, pressed, and often left in a tin until lunch, and it is one of the few sandwiches that improves a little for sitting, the paste settling into the crumb rather than weeping out of it. The whole point is that a very small amount of something concentrated turns bread and butter into a meal.
The variations are the rest of the paste row, each the same engineering met with a different protein in the jar. The salmon paste sandwich carries the potted idea into another fish; the bloater paste sandwich takes it to the strong smoked end; the beef and chicken paste sandwiches swap the shellfish for cured meat. The jarred crab paste itself, seen as a product rather than a built sandwich, is its own consideration. Each deserves its own article rather than being crowded in here.