Preheat oven to 400 degrees F (200 degrees C). Place a sheet pan on the lower oven rack.
Wash and stem grapes; squeeze grapes out of their skins. Save the skins and set aside. Place grape pulp in a large saucepan; mash a few at the bottom to release their juice. Cook over medium-low heat until grapes come to a full boil. Remove pulp from heat, and press through a food mill to remove seeds. Combine pulp and skins in a large bowl. Stir in lemon juice.
In a separate bowl, mix sugar, flour, and salt. Stir into grape mixture. Pour filling into pastry crust, and dot with butter. Cover with second pastry shell. Flute edges, and cut little slits in the top crust for steam to escape.
In preheated oven on sheet pan for 45 to 50 minutes, or until crust is brown and juice begins to bubble through slits in top crust. Allow to cool before serving.