I don't believe there is a verifiable way to conclusively prove what STX stands for.
I have seen many threads like this in the past, and it always comes down to what people 'think'.
I've asked at the dealership myself, and of course they don't know, but that's not saying much.
If it does have an official meaning, perhaps someone else will know, I wouldn't mind knowing myself, but I don't think it has any meaning.
Now turn off your computiner and go to bed, it's 3am