re: Can you show me an early reference?
Online the only exact available reference is this one from 1951:
"Politically correct"
To give big loans to politically correct companies and individuals
This was used to describe good leftist companies in bed with the Truman administration's policies.
Stranger in a Strange Land was published ten years later.
Here TIME writers use the phrase "political correctness" in 1947,
in reference to requiring people to hold a certain political viewpoint.
However, my personal library shows the phrase "politically correct" in use already by mid-1944 to describe young college activists and "grayhairs" (professors) with a socialist, peace-minded agenda in the New York Times Magazine. It may even be older than that, but that's the earliest I have encountered.
You'll just have to take my word for it.
;-)
The phrase has been around a LONG time, even if it only acquired public conscience late in the 20th century.