The buying situation left you with the impression this was definitely a named selection? I ask because I've seen what appeared to be unselected seedlings (with less built up flowers such as yours has) used by commercial growers as root-stocks for tree peonies and allowed by final purchasers to grow up and flower beneath the scions (as often happens with grafted woody ornamentals). So if somebody had a tree peony fail and kept the stock used for it, let that develop it seems a third party could end up with one of those on some occasions.