The one aspect about First Day Covers that should appeal to collectors is that when the stamps are franked, they are done carefully, knowing that the FDC will be going to a collector.
Consequently there is usually a minimum amount of the franking or postmark on the stamp. This is a much better way to add used versions of stamps to your collection than removing them from parcels or letters which have been run through postal machines with no care paid as to where the postmark will land.
The Penguin cover illustrated is a commemorative but the positioning of the postmarks doesn't indicate the care one would get with a First Day Cover, which is a pity.
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