One of my dad's friends owned an ex-military jeep back in the 60's and 70's.
He used to take it on closed land and property all the time to hunt and fish in the mountains in Southern California.
He had grown up there all of his life and he despised the land closures.
He stenciled this on the sides of his jeep:
USC 123-456-7890 MAR 24
After that he was able to get into most of the closed off areas with no problems, and if he was stopped no one ever said anything.
USC = United States Citizen
123-456-7890 = social security number
MAR 24 = Birth Date