This is coming up at RR Auctions Remarkable Rarities auction on 11 September.
Given the reputation of the auction house and the high profile nature of this particular auction, I cannot believe it isn't real but the neatness and form of the signature (particularly the "Marilyn" part) certainly seem a little unusual to me. The inscription looks much more typical but it is also perhaps unusual that Marilyn has used the full name of the recipient. I always thought that these high quality double weight photographs were usually used for close friends.
Has anyone seen similar signatures or know who Lois McCann was?