What does the part in quotes mean?
Three generations had nailed bright metal signs—Nehi Grape Soda, Royal Crown Cola, Camel Filters, and "twenty years’ worth of North Carolina automobile license plates"—all over the shack, and that burst of color could be seen from the sea through all but the thickest fog.
Edit:
Does "twenty years’ worth" mean "they had been collecting license plates for 20 years"?