Still pulling the thread. So far, all I find about this pizzeria is a mention in a Facebook group dedicated to Bajos de Argüelles. I'll keep asking to see if I can get any photos of the place or its food. It had a rather eclectic menu ranging from various pizzas, zíngara-style entrecôte, Hungarian goulash to Neapolitan spaghetti, Viennese schnitzel, or knuckle with sauerkraut and mashed potatoes...

Otherwise, the absence of the prefix 91 and the use of the old postal designation "Madrid-15" indicate that this box was printed before the widespread adoption of modern postal codes, probably between the late 70s and early 80s.