Lay out a sheet tray, place 20 bread slices on it, if desired, match up 20 more bread slices, reserving ends, and any pieces, with large holes in them, for something else, then set the 20 matched bread slices aside, in order to assemble the sandwiches, keeping only 20 slices on the sheet, making four rows of five.
Put 1 teaspoon of dressing on each bread slice, I put it in the middle, and don't spread it out, because it's going to squish, when the fillings are added.
Stack the ten Swiss cheese slices on top of each other. Cut them in four (that makes 40). Put one piece of cheese, on each dressed slice, follow by one whole slice of deli corned beef, followed by some sauerkraut, and then the remaining 20 cheese slices. If a saucier sandwich is desired, a little more dressing may be placed on the interior, of each bread slice, before finally assembling the sandwich.
Brush the bread slices evenly with half of the melted butter. Turn each sandwich, and brush the remaining butter on them.
Bake at 375 for 10-12 minutes, serve at once, accompanied by a bowl of dressing, for dipping, on the side.
Note: the sandwiches are also good about 20 minutes after coming out of the oven, when they are still slightly warm.
Another note: if a very small party appetizer is desired, each sandwich may be cut two, easily, but they will need to be transferred to an appropriate surface, in batches, to do it, before being transferred to plates, for serving.