1/2 cup butter
2 T. grated lemon rind
1/2 cup lemon juice
1 1/2 cups sugar
3 whole eggs
3 egg yolks  

In top of a double boiler, melt the butter.  Stir in lemon rind, juice 
and sugar.  Beat the eggs and yolks together until thick; blend into 
lemon mixture.  Cook, stirring constantly, over hot water until mixture 
is very thick.  Chill before using.

Source: 1957 Woman's Day Collector's Cookbook