I don't think any four-letter words were used as freely in the 60s. the "Lady Chatterley " trial was at this period; it was ground-breaking in that the book contained various "unacceptable" words, though used in their proper context, not as swear-words, and the publishers won their case and got the go-ahead to publish it.
But one of the issues of the trial was class distinctions ; one barrister asked, and was made fun of, whether it was a book you'd want your wife or servants to read. The implication was that the words being fought over were familiar only to the lower orders. If that was the case, and it wasn't inevitably, then the "lower orders", meaning working-class men, would have used them among themselves but not in the presence of women and children.
Therefore I think it entirely possible that Jack and Ennis knew the words and had heard them used, might use them themselves, but only when there were no women and children, or indeed unfamiliar males, specially those of higher status, around. And rather like D H Lawrence, Jack if he uses the words in this scene uses them only in their literal sense to indicate what he wants Ennis to do.