Based on the wording of your post I don't think you understand what a fin is. A fin, or finning as it's more commonly called, is nothing but excess metal that squeezes up between the die and the collar when a coin is struck with too much pressure. Finning, while common at times, is an accident. It is by no means intentional or by design on either Proofs or business strike coins. But it is sometimes found on both Proofs and business strike coins. There is however a diagnostic that is unique to Proof coins - that being that they always have sharp, squared off corners on the rim. Business strike coins on the other hand have rounded or beveled corners on the rim.
Why not just ask in the thread you started yesterday? I'm assuming this is in reference to the coin you posted already.
