The SIDO isn't a productin line build number though nkxr6, is just a dealer order number that can be used to trace that cars whereabouts.
If you order an XR6 with premium sound, sun roof etc and one or both are in short supply your car will be put back, mine for instance was put back by about 405 cars from its original assembley line build number.
Once mine got onto the assy line, it took about 2 weeks before it arrived at the dealer here in Perth.
The dealer can tel lyou exactly what stage the car is at, metal, trim, holding yard @ Ford, holding yard @ Tickford (still called Tickford on the system!!) etc