The SX is a step up from the MX with the additions of a second tone knob and upgrading to the Bendmaster Deluxe. There are several examples of SX labled necks on seemingly MX featured bodies. I've seen FT and Deluxe bridges on otherwise identically featured/labled guitars. The retail price difference was $349 to $460 on the August 1986 pricelist! They are obviously identical all maple necks and bodies.

Also evidently they started putting Bendmaster Deluxe on more and more models so the later ones seem more likely to have the Deluxe.

Westone Spectrum MX (X139) $349
Spectrum SX (X140) $440
Spectrum LX (X198) HSH bolt-on $575
Spectrum FX (X199) $659 - $699 depending on color, HSH set-neck OR neck-through for different years

At first glance, this looks like the Spectrum MX. There are a few differences.

The SX has a different trem (Double locking Bendmaster FT with fine tuners) and 2 tone controls. It also had a push-pull tone pot for phase reverse in addition to the coil tap present on the MX. The pickups are also different (Magnaflux IV single coils with a Magnaflux II UBC humbucker)

First appeared in the 1985 UK catalogue, available in Transparent Red or Transparent Blue (With Hard Maple neck and fingerboard), it was also available in red and black, both colours with a Rosewood fingerboard.

First appears in US pricelists in Jan 1985 at $399.50 for the Trans Red, Trans Blue and Black finishes (Red finish not mentioned), rises to $460 in August 1986. Last appears in the Sept 1987 pricelist (Only the Trans Red finish is available at that time)

Oddly, it doesn't appear in the April 1987 pricelist, and none of the US pricelists show the (non-transparent) red finish