Combine the sugar, flour & salt in a medium sauce pan.
Whisk in the milk; cook & stir until thick and bubbly.
Reduce heat and cook 2 more minutes, stirring constantly. Remove from heat.
Beat the yolks slightly in a small bowl, then add about 1 cup of the hot mixture, stirring until well mixed.
Stir the yolk mixture into the saucepan, and bring back to a gentle boil for 2 minutes, stirring constantly
Remove from heat; add butter, rind and juice.
Fold in the sour cream.
Pour into baked shell and chill.
Top with whipped cream or cool whip when serving.