Chiffon pies with gelatin date to the 1920s, but were popularized in the 1940s.
From a box sold in Canby, Oregon.
Lemon Chiffon Pie
1 c. sugar
3T. lemon juice
1/2 T. Knox Gelatin in 1/3 c. water (cold)
Separate eggs. Beat yolks; add 1/2 c. sugar, lemon juice, and lemon rind. Cook in double boiler until thick and creamy; take off stove and add gelatin in water.
Beat egg whites until almost stiff. Add 1/2 c. sugar and beat again.
Fold whites into cooked mixture and stir all well. Pour into baked, cooled shell; let set 3 hours. Serve with whipped cream.