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.