Primary or Secondary Sources?

Westward Expansion Stamps

3-cent Transcontinental Railroad stamp
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50-cent Trans-Mississippi Western Mining Prospector stamp
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3-cent Kansas Territorial Centennial stamp
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$1 Black Western Cattle in Storm stamp

Are these four stamps primary or secondary sources for 19th century Westward Expansion? How do you know the difference? Remember to look closely at each stamp and the information provided so you can draw a sound conclusion for each stamp.