The dressed crab sandwich is defined by an act of restraint that happens before any bread is involved. A crab is picked into two distinct meats: the pale, sweet white from the claws and legs, and the soft, rich, slightly bitter brown from the body. Dressing the crab means binding each one separately, the white kept loose and barely held, the brown worked smooth with a little lemon and seasoning, and then returning them to the cleaned shell in their own zones. By the time it reaches a slice of bread the sandwich has already been made; the bread is the last and least important decision, a plain carrier for something that has been balanced on its own terms.
The craft is in the ratio of the two meats and in keeping moisture under control. White crab on its own is delicate to the point of being thin, and brown crab on its own is too strong and too rich to eat in quantity, so the dressed version exists to set them against each other in measured proportion: enough brown to give depth, enough white to keep it sweet and light. The bind is deliberately minimal, a touch of mayonnaise rather than a coating, because crab handled too hard turns to a uniform paste and loses the distinction the dressing was meant to preserve. Soft white or brown bread, buttered to the edge so the crumb does not go wet, holds the meat in a thin, even layer; a sharp acid note from lemon or a turn of pepper does the only seasoning the sandwich needs, since the crab carries the rest.
The variations stay close to the coast and to the same principle of leaving good shellfish alone. The brown-meat-forward version reads almost like a potted spread; the all-white build is leaner and sweeter and closer to a prawn sandwich in feel; the Cromer and Devon traditions each defend their own local crab and their own degree of dressing. Those deserve their own articles rather than being crowded in here.