This was covered in the 2008 rule change explanation document that was posted here for a year. Unfortunately, we are on to the 2009 rules now, so that document is gone from the site. Here is the relevant section:
Question:
How do I tell if my stock 3-point belts are out of date? If the car is over 20 years old do I
have to replace my belts with new ones?
Answer:
This rule reads:
Factory belts more than 20 years old or visibly frayed or damaged at any age, must be
replaced with new belts. This is a little fuzzier as factory belts have no date coding. As a
rule 3-point belts must be in good condition. If the belts show signs of wear (fraying,
curling of edges, twisting or sticking inertia reel) they will be disallowed, regardless of the
age of the vehicle. Newer belts and replacement belts have a few visual clues that tell you
their age. First, there is a tag where the belt attaches to the floor. That tag should be
black, not white. On the other end of the belt is the buckle. (The chrome flat part that
goes into the receiver). This part should have hole in it. If it is actually a round hole it is
definitely out of date.
The hole should be a small rectangle like the photo below.
If the belt is even newer it will have a large rectangle or square hole in the center.
HTH,
TT