The dry-cured bacon butty is separated from the ordinary bacon butty by what is missing from the pan. Most supermarket bacon is wet-cured, pumped with brine, so it leaks a milky white liquid when it hits heat and steams rather than fries until that water has gone. Dry-cured bacon is rubbed with salt and left to lose moisture instead of gaining it, so it goes straight to a hard fry: the fat renders clean, the edge crisps fast, and there is no white seepage to boil the rasher soft. The defining fact of this butty is that the cure, decided long before the bread, is what makes it fry the way it does.
The craft is heat and grease management against bacon that behaves differently. Because dry-cured rashers shed fat rather than water, they crisp quickly and concentrate their flavour, so the cook is short and hot and the roll has to be ready to take that rendered fat the moment the bacon comes off. The bread is a soft floured roll, yielding enough to soak a little of the hot dripping without disintegrating, and buttered so the salt of the bacon is bridged across to the wheat rather than sitting on top of it. The sauce question, brown or red, is applied inside in a measured stripe so it does not run, and it is used sparingly here because dry-cured bacon is already the loudest thing in the sandwich and a flood of sauce only buries the cure it was built around.
The variations all turn on the cure and the smoke rather than the assembly. The unsmoked dry-cure is cleaner and more directly porky; the oak- or beech-smoked version adds a second savoury layer on top of the concentrated salt; the streaky and back cuts change the fat-to-meat ratio and so change how hard the rasher crisps. Those deserve their own articles rather than being crowded in here.