Place egg whites in a small mixing bowl, let stand at room temperature for 30 minutes. Meanwhile, in a large saucepan, combine the pudding mix, milk, and sour cream until smooth. Cook and stir over medium heat until thickened and bubbly, about 5 minutes. Reduce heat, cook and stir 2 minutes longer.
Remove from the heat, gradually whisk 1 cup hot filling into egg yolks, return all to the pan. Bring to a gentle boil, cook and stir for 2 minutes. Remove from the heat. Gently stir in lemonade concentrate, keep warm.
Add lemon juice and cream of tartar to egg whites, beat on medium speed until soft peaks form. Gradually beat in sugar, 1 TBSP at a time, on high until stiff glossy peaks form and sugar is dissolved.
Pour warm filling into pastry shell. Spread meringue over filling, sealing edges to pastry.
Bake at 350 degrees for 15-20 minutes or until meringue is golden brown. Cool on a wire rack for 1 hour. Refrigerate for at least 3 hours before serving.
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