Potato Lemon Pie Filling

From 1911:

Make a rich crust, bake, let cool, then add the following filling: The grated rind and juice of one lemon, a grated potato little larger than a lemon; one cup of sugar, one tablespoon of flour; mix the potato with the lemon; mix the sugar and flour together, then add the lemon and potato. Pour over this one cup of boiling water, cook until the potatoes are thoroughly done. Beat the white of one egg with one cup of sugar until thick and add the sugar slowly. The above is for one pie.