MX de Nations during the '70 and early '80s was run as a three person national team. A 125 rider, 250 rider & 500 rider. Three 40min motos ran. Moto 1 - 125 & 250, Moto 2 - 250 & 500, Moto 3 - 125 & 500. I can not remember which order the races where held. The scoring was 1 for 1st, 2 for 2nd etc. The team with the lowest score won. Sometime during that period the scoring rule changes so the team could drop its worst score. Thats my memory of it anyway. Belgium was the force in MX de Nations at the time. Well the seventies anyway.
Thats my recollection and am happy to be corrected.