Some of you may already know this, but for those who may not, it's possible to get pretty close to working out the exact month as well as the year of your car's manufacture.
There's a label on each seat belt at one end (usually the "long" end just before its attachment point to the car body), which says when the seat belt was made. Of course this assumes that your belts are the original ones, but that seems quite likely unless the car's been in a major accident.
On the front belts in my IS200, this just has the year, but on my rear belts it has the month too, saying the belts were made in July 1999. Since my car was registered in September 1999, I now know that it was made some time between July and September 1999 (since it's unlike to have started being made before the seat belts were made). Given it was built in Japan and the time to travel here etc, I imagine it was made primarily in August 1999 ?
Just thought I'd pass this on, perhaps not that useful but some might be interested