NEW YORK/LONDON (Reuters) – Pfizer Inc (PFE.N) is the mystery bidder for Swedish Orphan Biovitrum AB (SOBIV.ST), the rare disease specialist which disclosed this week that a potential buyer had made a preliminary offer, people with knowledge of the situation said. The Swedish company, known as Sobi, has a market value of 35.8 billion Swedish crowns ($4.3 billion).