I am not a Canadian nor PEI specialist, but I saw this and thought it was an interesting item.
Besides welcoming your general comment about this item, I was wondering if the plate proof was pulled contemporaneously to the creation of the stamp plate or if such items were pulled much later for some reason by the printing organization or the Post Office.
Also a general price range of a single plate proof and the entire sheet as well as price/interest comparison between the plate and die proofs. Can I assume that plate proofs are the stepchildren of die proof in this area as is the general case with early US material?
Knowing the current owner, he has two wonderful dogs and as such the curved line at the upper left likely was from the Weimaraner.