The salmon 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. Salmon paste, a cooked and pounded fish spread concentrated to survive months sealed in a cupboard, 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 spread built for shelf-stability is so salty and so strong that a thin smear is already a full mouthful and a thick layer is uneatable. It is the economical fish sandwich, salmon between bread without poaching or smoking any.
The craft is the scrape and the butter under it. Butter is not optional lubrication here; it is the carrier that drags the paste's salt evenly across the slice and stops a thin pink smear reading as a stripe with bare bread on either side, and a sandwich made without it tastes of one sharp band and nothing else. The paste holds the faintly oily, pink note of salmon but has no flake left in it, so it brings flavour and no texture, which is why the bread is always soft and plain: a crust with real chew 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 sits well, the paste working 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 made into sandwiches, each the same engineering met with a different protein in the jar. The crab paste sandwich carries the potted idea into shellfish; the bloater paste sandwich takes it to the strong smoked end; the beef and chicken paste sandwiches swap the fish for cured meat. The jarred salmon paste itself, read as a product rather than a built sandwich, is its own consideration. Each deserves its own article rather than being crowded in here.