"Rarest of the rare"
An interesting phrase, one would assume it be married to an intimate, and wide ranging knowledge of pre existing , similar examples.
As soon as I personally see "rare" suggested in Philately,
red warning signs emerge.
A common descriptive found at eBay
"Establishing a rarity level for any collectible is always difficult and controversial. Philately and any of its related branches are not an exception.
Personally, it has always been hard to believe in a rarity estimation stated by a merchant, or derived by one, because is very hard to have an unbiased judgement when your own meat is at stake."
Author : Will Castillohttps://formularcards.com/relative-rarity/