This Homestead Act stamp is a secondary source. The numbers on the right side of the stamp show the dates 1862 and 1962 - a big clue to the historian that the stamp was created in 1962 commemorate the anniversary of The Homestead Act.
Paying close attention to every detail is important, especially with this kind of stamp. It is not only monochromatic (one color) but low cost - qualities similar to a stamp that could be created in 1862. That's why the dates are important clues in determining whether it is a primary or secondary source.