Divide slices of bread between four plates and spread with butter all the way to the crust. Arrange the cucumber slices over the butter, overlapping in a fish scale pattern.
In a medium mixing bowl, vigorously stir the eggs, mayonnaise, sour cream, and salt until combined and the egg yolks have broken apart and become part of the sauce.
Spoon the egg salad over the cucumbers, then arrange the pickled herring evenly across each sandwich. Top with watercress and garnish with fresh dill.