The morning roll sandwich is named for its bread, and in Scotland the roll is the sandwich. A morning roll is a soft, dense, floury bap, baked to be eaten the day it is made, with a tender pale crust dusted in flour and a close, slightly chewy crumb that compresses rather than crumbling. Whatever goes inside it, the roll is the fixed identity and the filling is the variable, which is the opposite of how a sandwich named for its filling works. Ask for a morning roll in Glasgow or Aberdeen and the bread is understood before anything else is decided, because the roll is the thing that makes it that sandwich rather than any roll with the same filling in it.
The craft is in the bread's structure and its short life. A morning roll is soft enough to yield to a bite without tearing and dense enough to absorb a little fat or moisture without going to paste, which is exactly the property a hot, greasy breakfast filling needs in a carrier. It is floury on the outside on purpose, a dry dusting that keeps the crust soft and gives the hand something to hold that is not slick. Because it stales fast it is filled and eaten the same morning, often within an hour of baking, which is why it is a morning roll and not simply a roll. Butter to the edges is not optional: it bridges the salt of a savoury filling to the plain wheat of the crumb and seals against moisture for the short time the sandwich exists.
The variations are whatever the roll is asked to hold, and the canon is a Scottish breakfast. Bacon, Lorne sausage, a fried egg, black pudding, a tattie scone, the link sausage, each is folded into the same floury roll and named for the filling within a bread that is taken for granted. Each of those deserves its own article rather than being crowded in here, because the interest there is in the filling, while here the interest is the roll itself.