Combine sour cream, 1 3/4 cup of the sugar, flour, egg yolks and raisins in a heavy saucepan or top of double boiler and cook, stirring constantly, until thick and glossy. Pour in baked pie shell and chill. Put egg whites into a metal mixing bowl over a pan of hot water. Stir in remaining 1/2 cup of sugar and salt. After a few seconds, when whites are tepid and sugar is dissolved, whip until they stand in peaks. Spread across pie, touching entire rim of crust, and place under broiler until just barely browned (less than a minute.)