In a saucepan, combine sugar, flour, and salt. Gradually stir in milk. Cook and stir on medium heat until thickened and bubbly. Reduce heat, and cook and stir 2 more minutes. Remove from heat.
Beat yolks slightly. Gradually stir 1 cup of mix into yolks. Return yolk mixture to saucepan, and bring to gentle boil. Cook and stir 2 more minutes. Remove from heat, and stir in butter, peel, and juice. Fold in sour cream.
Pour filling into baked pie shell, and cool. Top with whipped cream if desired.