These started to pop up from Day 36 in Year 35 (23th April 2008) on.
from Mr. Jonas' blog:
National Treasures
2008-04-23 15:00
There are items so special, so filled with historical significance, so valuable that we consider them National Treasures.
Lie, cheat, steal, or open up your bank vault to get your filthy paws on them because there won't ever be more than one of each kind.