In the top of a double boiler, beat the egg yolks well. Combine the flour, sugar, 1/8 teaspoon salt, allspice, and mace and blend into the egg yolks. Add the 1/4 cup cider vinegar and mix, then add the warm water. Simmer over boiling water for 25 minutes, or until the mixture is thickened; the water should just reach the bottom of the top pan.
Stir in the butter and cool slightly, about 15-20 minutes, without stirring. Pour the warm filling into the baked shell; set aside to cool.
Preheat the oven to 325 degrees F (165 degrees C). In a large mixer bowl, beat together the egg whites, 1/4 teaspoon salt and 1 teaspoon cider vinegar until soft peaks form. Gradually add the 6 tablespoons of sugar, 1 tablespoon at a time. Continue beating until stiff peaks form; sprinkle in the cornstarch just before the beating is completed. The peaks should not topple over when the beater is raised.
Spread the meringue over the filling all the way to the edge of the pastry. Bake for 15-18 minutes, or until the meringue is golden brown. Cool the pie completely and then refrigerate.