This comes from a promo recipe card (paper) that I believe was distributed by the American Dairy Association (ADA), date unknown. Recipe is typed below along with a scanned copy.

Holiday Buttercorn
Yield: Approx. 20 cups

16 cups popped popcorn
1 cup pecan halves
1 cup blanched whole almonds
1 cup walnut halves
2 cups firmly packed brown sugar
1 cup (2 sticks) butter
1/2 cup dark corn syrup
1/2 teaspoon salt
1/2 teaspoon baking soda

Preheat oven to 250°F. Put popcorn and nuts in a large roasting pan. Keep warm in a 250° oven. Put brown sugar, butter, syrup and salt in a large saucepan. Bring to a boil, stirring constantly. Stop stirring and cook to 250° on candy thermometer, about 5 minutes. Stir soda into carmel. Drizzle over popcorn; toss to coat. Bake in 250° oven 5 to 10 minutes. For crispier corn, bake 15 to 20 minutes.