In a recent answer about GM-ming in D&D, I had to resort to a certain "universal" rule I'm familiar with: “the GM is always right”. Upon this doppelgreener correctly pointed out that it is not universal about all games but a/the core of the D&D mentality. In the chat then kviiri pointed out that "The GM is always right" is actually a pretty crap rule by itself. This "universal rule" also might have other names, like "Golden Rule" or "Rule 0", but that is not part of the question.1
Now I wonder: Where and when did "The GM is always right" or rather "The GM has the final say in any question" actually got codified in an RPG for the first time?
This is not about when the GM is asked to improvise or to handle something akin to his own GM Fiat; it is about where such a statement was first mentioned in a print product explicitly or implicitly. It has been used in various variants as far as I know, and to various extents. Some examples of these "The GM is always right" statements:
Hc Svnt Dracones, p5:
The Guide […]They also have the final call
on rules disputes and typically control
what stays and what goes if something
seems out of line.
Sengoku Revised Edition, p7:
The GM Rules – This is not a democracy. The GM is the boss. You should feel free to ask questions, but when a ruling is made, accept it.
And in Paranoia: Troubleshooters (2009, 25th Aniversary Edition) p.40 (and in this case the emphasis is not added):
GM Rule #1. You are IN CHARGE. You are ALWAYS RIGHT.
We give you these rules as guidance. Use them when you do not know what you’d like to have happen in the game. When you do know, ignore them. We have tried to make the rules as helpful and powerful as we can, but if you don’t like a rule, the rule is wrong. Good rules help a lot but bad rules were made to be broken, tortured, lobotomised and summarily executed. Dice are handy for giving players the illusion they control their destiny. This is valuable but roll your dice out of the players’ sight, behind a screen. If a die roll gives you a result you don’t like, the die is wrong. Change the result to the number you want. You can dock the die credits or beat it up, though in our experience this has little effect.
1 – I am fully aware that both Golden Rule and Rule 0 sometimes refer to "GM is right" and sometimes to "Have fun".