I wonder if the reason they were available for such a long time lies in the fact that they had been issued without gum. The missing gum meant they could only be used if there was a glue pot around, other stamps were easier to handle. At the same time they were less susceptible to the tropical climate. The #20 on Albert's cover looks pristine.
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http://www.philib.org/covers/pics_full/1920-12-27_154N_177b_M7_O91_20.jpg
I wonder if the reason they were available for such a long time lies in the fact that they had been issued without gum. The missing gum meant they could only be used if there was a glue pot around, other stamps were easier to handle. At the same time they were less susceptible to the tropical climate. The #20 on Albert's cover looks pristine.