Potato Eggs

From 1911:

Roast four or six large potatoes in the oven with their skins on; remove the insides, but keep the shape of the potato. Put the inside of the potato in a basin and add one and a half ounces of butter (according to the number and size of the potatoes) and salt and pepper to taste. Mix together until light and add either one or two well beaten whites of eggs and beat up all together. Fill the potato skins with the paste, first rolling each piece in beaten yolk of egg; then cook in the oven and serve as soon as the tops are well colored – ten to fifteen minutes.