But in auctions, it is obvious that if a cancel does not tie a stamp to a cover, the price someone pays will always be lower. When I bought the entire letter ( a printed death notice) shown above recently, no one else bid for it. I can see three reasons: it is not particularly attractive - the ink address has faded, there are creases; there is a brown smudge on the stamp; and the stamp is not tied to the letter sheet. The Finnish expert Rolf Gummessson thought the stamp belongs to the cover and gave a certificate in 1990, pointing out that the style of the cancel is that found on stamps used from Lovisa. And if the stamp had been added, there would surely be some indication on the cover itself of postal charges paid or due on reception but there aren't. There is just a receiver boxed ANK on the back and a typical Finnish distribution mark "1" in the top right of the cover.
In the case of this cover, I can't really see any collateral evidence to persuade anyone that the stamp belongs. The little brown stain bottom right of the stamp seems to extend onto the letter sheet but that does not really help - the stain could have been created in many ways. Looking at the stamp from the back of the sheet does not offer any insight. And so on.
In the end, what you have here is a judgement call - Gummesson thought it OK no doubt in part because he had seen many Finnish covers like this franked with a stamp which is itself quite rare.