Some of the ores, or least commonly collected items in philately are stamp essays, progressive and plate proofs. For the same reasons, these can be sold for higher prices.
Generally, none of these should reach the philatelic collector, as they are only to be shared between the printer and the Postal Authority.
Occasionally at the dissolution of a company, these items maybe sold off.
The books will contain many more variations, essays and proofs.
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