I’m guessing Sally’s Spread is a soy margarine.
Why? Well, compare this to a classic scalloped potato recipe, like the one from Fannie Merritt Farmer’s The Boston Cooking-School Cook Book (scroll to page 311):
Wash, pare, soak, and cut four potatoes in one-fourth inch slices. Put a layer in buttered baking-dish,sprinkle with salt and pepper, dredge with flour, and dot over with one-half tablespoon butter; repeat. Add hot milk until it may be seen through top layer, bake one and one-fourth hours or until potato is soft.
You’ll notice that the missing elements in this recipe are milk and butter. Given that so many of the recipes in this box are vegan, I’m guessing the water is replacing the milk and that Sally’s Spread is a soy-based margarine.
From a box sold in Chicago, Illinois.
Scrub potatoes and remove bad spots; do not peel. Slice in layers into greased baking dish. After each layer dot with Sally’s Spread and sprinkle gently with salt.
Add water to half fill baking dish. Bake at 350 deg. one hour or until tender.