I agree with Brummie; it has the "look" of a 560S. By "look", I'm centering on the head tube length and the "openness" of the main triangle, coupled with the length of stem extension. There's a bit more to support that assumption...
Looking at the xTc brochure (
http://web.archive.org/web/20060321060909/http://www.sjscycles.com/thornwebsite/thornpdf/ThornxTc853BroHiRes.pdf , pg. 4), the 55cm top tube length the seller reports would put it spot-on as a size 560S (short top tube, intended for use with drop handlebars).
The seller reports a 52cm/20.5in seat tube. If he measured
c-t, that would be awfully close to the 50cm
c-c Thorn lists for the 560S, so more evidence there.
The Thorn xTc brochure refers to the 560S frame size as having a "Frame Number" of 94. That number might appear somewhere on this vintage xTc, or the serial might start with 560S as it would have on later Sherpa Mk2s.
Lots of fun to speculate! It appears the incorrect size listing might be the reason why the bike hasn't sold. Might be a good opportunity to snap up a relative bargain on a lovely-looking bike if you're "in the know".
Best,
Dan. (...who is having a lot of fun with Bicycle Forensics today)