The mystery:

Are these William Rennie perfins legitimate?

The solution:

Not quite...maybe.

In addition to his other interests, William Rennie was a stamp collector and dealer. With that background, the stamps purchased and used by Rennie's Seeds were not completely random; therefore, these might be philatelic. However, there is no evidence that Rennie perforated and mailed these particular imperforated stamps to create a collectable piece, so they are not completely philatelic...maybe.