The 442nd Siege Battalion was a unit of clone troopers within the Grand Army of the Republic. As their name implied, the 442nd were skilled in besieging enemy fortifications. They were equipped with AT-RT walkers and ISP speeders with green markings. During the Clone Wars, they were sent to aid in the capture of Cato Neimoidia, a key Trade Federation stronghold.
It is possible that this unit takes its designation from the 442nd Regimental Combat Team, a highly-decorated unit of the US Army consisting mostly of Nisei volunteers. These Japanese-American soldiers earned many honors fighting in brutal close-quarters and mountain combat in Italy during World War II. (source: wookiepedia)
The 442nd was a self-sufficient force, and fought with uncommon distinction in Italy, southern France, and Germany. The unit became the most highly–decorated regiment in the history of the United States armed forces, including 21 Medal of Honor recipients. The motto of the 442nd Regimental Combat Team was “Go for Broke.”
Here's another Sideshow Collectibles clone trooper from their Star Wars Militaries series - the 1/6 scale 442nd Siege Battalion Clone Trooper has the same armor as all the other earlier released clone troopers including the Phase II helmet but with a different color scheme.
You can pre-order this Sideshow Collectibles Star Wars 1/6 scale 442nd Siege Battalion Clone Trooper 12-inch figure at TFH till it closes on 11-12-2011. Launch date is expected to be in the 2nd Quarter 2012.