This is a Portal sign for the Mark Twain National Forest.
You're entering the proclamation boundary of the forest. The forest has many in holdings or private properties. The drive on US 160 has many homes along the way. But you are in the boundary of the forest. If the forest trades or buys property it will be within the proclamation boundary.
So why is this a new sign? It's the newer style and is made of Plastic, the older ones are wood. This sign was replaced after the old one was burned down, believed to have been a case of arson. As far as I know they did not find the person who burned it down. We have a wood sign on the Poplar Bluff district that is at least 40 years old. I age it because the forest has not been known as the Clark National Forest since the 1970's. That sign is located by a cache called "Mark not Clark" (Was the sign is down the photos are on the Geocache page.).
Happy Hunting!
"Please be aware that collection of historic and prehistoric artifacts or the destruction or defacement of archaeological sites on federal lands is prohibited by the Archaeological Resources Protection Act of 1979, (16 U.S.C. 470aa-470mm) and the Antiquities act of 1906 (16USC 431-433). First time offenders can be fined up to $20,000 and imprisoned for up to one year. Second time offenders can be fined up to $100,000 and imprisoned for up to 5 years." — Forest Archeologist