Someone was asking for a recipe for this. While cleaning up some
papers, I found my dh's grandmother's (one of) cookbook. It was called
"Favourite Recipes" put out by the Woodstock Ontario Home and School
Council. I estimate the date to be somewhere in the thirties.

As written:

Rhubarb Custard Pie- 2 1/2 cups cut rhubarb, 2 egg yolks,1 cup sugar,2
tablespoons flour, 1 tablespoon melted butter. Beat egg yolks to a
thick froth; gradually add the sugar, flour and butter. Add rhubarb,cut
in one-half inch pieces. Pour into uncooked pastry shell and bake like
custard pie. When baked, top with a meringue made of the egg whites. -