In a small bowl cream the butter and stir in the anchovy paste, the scallions, and salt and pepper to taste. Spread one side of each of 3 slices of the bread with some of the scallion butter and top the buttered side of one of the slices with an overlapping layer of the radishes. Top the radishes with another slice of the buttered bread, buttered side down, and butter the top of the slice. Top the butter with an overlapping layer of the radishes and top the radishes with the remaining slice of the buttered bread, buttered side down. Press the sandwich together gently, trim the crusts, and cut the sandwich lengthwise into thirds. Make sandwiches with the remaining bread, butter, and radishes in the same manner. The sandwiches may be made 2 hours in advance and kept wrapped in plastic and chilled.