Combine flour, baking powder, and salt. Set aside. Pour condensed milk into meduim sized bowl; add lemon rind and juice. Stir until smooth. Set aside. In a large bowl, cram butter and sugar. Beat well. On lowest speed, add dry ingredients, beating until blended. Mix in the oats. Sprinkle a bit more that half of the oat mixture evenly over the bottom of a buttered 9 x 13 inch pan. Pat to make a smooth firm layer. Spoon lemon mixture over oat layer and spread to make a thin, smooth layer. Sprinkle remaining oat mixture evenly over lemon layer. Bake in the lower third of a preheated 350 degree oven for 30 to 35 minutes until lightly colored. Cool in pan; then refrigerate for 1 hour before cutting into squares. Store in refrigerator.