Seen from the bread rather than the jar, the fish paste sandwich is governed by how little of the filling you are allowed to use. Fish paste is a cooked, pounded, shelf-stable spread, white fish or a mix reduced and salted hard enough to survive months sealed in a jar at room temperature. That preservation is the whole story. To keep without refrigeration the paste is concentrated and salted to a point where it is overpowering on its own, so the sandwich runs on an inverted ratio: the paste is the thing you spread least of, because a thin scrape is already a full, savoury mouthful and a generous layer is simply too much. This is a different family from the fried fish sandwiches entirely. There is no coating, no heat, no patty, only a smear and the discipline to keep it thin.
The craft is the scrape and the butter under it. Butter spread to the edges of soft white or brown bread is not optional lubrication; it is the carrier that drags the paste's salt evenly across the slice and stops a thin film reading as a sharp stripe with bare bread on either side of it. The paste keeps a faint cooked-fish note but has no flake or texture left in it, so it brings flavour and nothing structural, 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 like the other potted sandwiches it settles rather than weeps, the paste sinking into the crumb instead of sliding out of it. The 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 shelf, each the same engineering with a different jar. The salmon paste sandwich carries the idea into a sweeter, oilier fish; the bloater paste sandwich pushes it to the strong cold-smoked end; the crab paste sandwich brings shellfish to the same potted logic; the beef and chicken pastes leave fish behind entirely. Each deserves its own article rather than being crowded in here.