This makes the following quote from Sprigge, which Kripke uses in N&N (p. 111), particularly interesting. It seems to provide a clear case of a philosopher confusing subjunctive necessity, on the one hand, with either indicative necessity or apriority on the other hand:
The anti-essentialist says that there would be no contradiction in a news bulletin asserting that it had been established that the Queen was not in fact the child of her supposed parents, but had been secretly adopted by them, and therefore the proposition that she is of Royal Blood is synthetic. In this way the anti-internalist parries the argument of the internalist by suggesting with regard to each proposed internal property of the particular in question, that we can quite well imagine that very same particular without the property in question. For a time he is winning. Yet there comes a time when his claims appear a trifle too far fetched. The internalist suggests that we cannot imagine that particular we call the Queen having the property of at no stage in her existence being human. If the anti-internalist admits this, admits that it is logically inconceivable that the Queen should have had the property of, say, always being a swan, then he admits that she has at least one internal property. If on the other hand he says that it is only a contingent fact that the Queen has ever been human, he says what it is hard to accept. Can we really consider it as conceivable that she should never have been human? (Sprigge (1962), p. 203.)
It seems pretty clear that here Sprigge takes the possibility of the bulletin - the possibility of finding out that Elizabeth II is not actually born of royal blood - as tantamount to it being the case that things could have gone such that she was not born of royal blood.
So, this quote makes it seem almost certain to me that someone - namely Sprigge - was actually confusing subjunctive necessity with either apriority or indicative necessity. Further questions are how widespread the confusion was around the time Sprigge wrote, and whether this was a relatively new thing at the time. Is it the case that, by the time Sprigge wrote the above but not for long before that, the notion of subjunctive or counterfactual necessity was "in the air", was salient, and so this sort of confusion is a relatively short term phenomenon occurring only in the lead-up to N&N (and shortly after, while people had yet to digest Kripke)? Or is the confusion something we can find much earlier evidence of?
Kripke, Saul A. (1980). Naming and Necessity. Harvard University Press.
Putnam, Hilary (1962). It ain't necessarily so. Journal of Philosophy 59 (22):658-671.
Sprigge, Timothy (1962). Internal and external properties. Mind 71 (282):197-212.