The panino con mantovana is named for its bread, not its filling, and that is the whole point of it. The mantovana is a Mantuan crusty loaf shaped with protruding tips, the so-called orecchie, ears that bake hard and sharp while the interior stays open and light. Pull one of those tips off and it shatters; the crumb underneath is soft and faintly sweet. As a sandwich the mantovana is split and filled, often with a single Lombard salume or a wedge of local cheese, but the bread is doing as much work as anything inside it: the contrast between a thin, almost glassy crust and a yielding centre is the texture the eater is actually buying, and a heavy or wet filling would only blunt it.
The craft is in respecting that crust. A mantovana is best the day it is baked, when the ears still snap and the crumb has not gone leathery, so the good version is filled to order rather than built ahead and left under glass. Because the loaf is dry and structured, the filling is kept spare and on the drier side: a few folds of salame mantovano or prosciutto, a slice of a firm cheese, a thread of oil at most. The bread is not buttered, because butter would soften the very surface that gives the sandwich its character. The fill stays modest so the bite leads with crust and only then arrives at the meat, the reverse of a soft roll where the bread surrenders first.
The variations are mostly about what local thing goes between the two halves rather than any change to the loaf. There is the salume reading with a Mantuan cured meat, the cheese reading with a wedge from the same plain, and the bare mantovana eaten with nothing but oil and salt, which is closer to the bread itself than to a sandwich. Each is the same shattering crust met by a single regional filling, and those deserve their own articles rather than being crowded in here.