Collector's cosmetics are rare, this is the most recently crafted. Craft numbers for Collector's items are generated by tf2tools.
A new id is created every time an item is traded/crafted/modified etc. Therefore, the rate at which this number increases gives an indication of player activity in TF2.
There are tens of thousands of possibe hat/weapon/effect combos. Our records indicate this is the latest 1-of-1 unusual.