It seems to be quite a challenge finding grades in World Cup and Euro games prior to 1986. Guerin Sportivo didn't publish any, nor did Italian newspapers such as Corriere della Sera and La Stampa. I have a few El Gráfico and Mundo Deportivo ratings, but that's it. Who knows more?
I'm guessing the 5 means something like "exceptional, world-class" and the 6 means something like "legendary, they'll be talking about this performance a century from now"?
Exactly. To translate it into the common 1-10 scale where 6=ok, 2=one og+a red card before ht and 10=Maradona or Messi at their very best, Mundo Deportivo's 0 would be a 3 grade and 6 a 9 grade.