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

None of the catalogues or pricelists I've seen mention any changes to the spec of the SX, but there must be at least two versions. Most of the ones I've seen  are as shown in the 1985 and 1986 catalogues, but I have seen two (Which have obviously not been modified) which only have a single tone control, and  no phase reverse switch. The pictures below are of one of these guitars which dates from mid-1986 (Serial number begins with 605, and is stamped on the neck plate).


Thanks to Ebay member meg1528 for the photos

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Buying tips - check the trem is complete, parts may be very hard to obtain. The pickups may not be the ones listed in the catalogues - see the UBC pickups page for details