So, interesting story...
I was looking at this and noticed that there are several photographs of the rulebook. On one of them, the section 'Playing with less than seven players' is visible.
It's very different to the post-1971 rulebook variants, in which you still play on a full board but people will control multiple powers. In these variants, parts of the board become impassable:
"With six players, eliminate Turkey and Bulgaria. Players may not move to these spaces. Fleets may move directly from the Black Sea to the Aegean Sea. With five, eliminate Russia and Rumania as well. With four, drop Austria-Hungary, Greece, and Serbia. With three, drop Italy and Tunis. Late comers take up the Great Powers in the reverse order in which they were eliminated."
So, these are officially the first Diplomacy variants - ever! Honestly they sound pretty terrible, but it's an interesting piece of Dip history nonetheless.