I agree with Jimmy: it's not a common term. To me, it sounds like King is trying refer to "Rosie the Riveter", but is avoiding that phrase because it is a bit cliched. So, he invented a similar term that everyone would understand, but which avoids the cliche. https://en.wikipedia.org/wiki/Rosie_the_Riveter

The context is that the man, his wife, and his wife's sister had operated a small airfield entirely by themselves. I have never heard the phrase "Socket Wrench Susie" before, but from context it is obvious that it means "female mechanic." In US colloquial English, a mechanic is sometimes referred to as "a wrench." I'm guessing that King constructed the phrase by analogy to some masculine equivalent, although "Rosie the Riveter" came to my mind, too. During World War II, "Rosie the Riveter" was a generic term for women working on mechanical jobs in war factories. A Google Books search on "socket wrench Susie" turns up nothing but its appearance in "The Night Flier," by Stephen King. It would be fairly normal to invent such a term, in a more-or-less friendly joking way. Also, King might very well have been using an expression that was strictly local to Portland, Maine.

I agree, too. It means that she's skilled at mechanical repairs and suchlike. The text is about a couple who seem to run a small airfield single-handed, with just some help from the wife's sister.

Stephen King invents language, here he seems to have coined a term for a female car mechanic. Please provide more of the text if you want a better answer.

Hi, the site Urban Dict also reports a vulgar meaning for "Socket wrench", not difficoult to imagine; I agree in any case that only the context might clear out what the author really means. Bye