What year did the five-inning rule to award starting pitcher credit for the win go into effect?

I’m in a disagreement with someone who says the rule that states that a starting pitcher much pitch at least full five innings (certain circumstances notwithstanding) to get credit for the win was adopted/changed in the 1980’s.

I think – and am fairly certain – the rule began much earlier than that, maybe even back to the '30’s, but can’t seem to find the date/year the rule went into effect.


Never mind. Found it. 1950.

Carry on.