Figure 4. A 25¢ Flag Over Yosemite plate number single coil stamp, Scott 2280a.
Figure 5. British coil stamps from 1912 to the 1950s have sideways watermarks.
Luminesence is the characteristic of the mineral zinc orthosilicate, which glows a yellowish-green when illuminated with shortwave ultraviolet light. The U.S first issue that used phosphored-coated paper was the Flag over Yosemite definitive coil in ...
The smooth or uneven appearance is not an indicator of type. The tagging can be carefully scraped off without damaging the paper. Surface Phosphor Tagging began with the 25c Flag Over Yosemite, Scott #2280.
 See also: Flag, Used, Catalog, Cover, Stamp
  
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