I don't know where Gram got this recipe. What I know is that it's one of my Dad's favorites. And that my mother went to her to make sure she got it written down. Except that she didn't really write the instructions down. My mom may be gone, but, luckily, I have an aunt that also knows this recipe. I made it for Father's Day and received the much coveted, and hard-won, "as good as your grandma and mom made it"!
Combine first four ingredients. Mix in the rhubarb until well combined.
Spray a pie pan with non-stick cooking spray. Lightly sugar the pan (it's the same technique as flouring a cake pan, but using sugar). Lay one of the pie crusts into the pan, being sure to gently press into bottom and around the outside. Pour rhubarb and custard into pie plate.
Place second pie crust on top. Fold the upper edges underneath the bottom crust, pinching in your favorite pattern to seal the pie together. Place vent cuts on the top of the crust in whatever pattern you choose (my Gram's signature design was the four card suits cut out and placed elsewhere on the top. My signature design is a winking sun face).
The pie may overflow during baking, so place something under the pie plate to catch the dribbles. Bake in a pre-heated 425*F oven for one hour.