Ressler's private investigation is certainly taking us further into the 'Red-is-really-Katarina-with-a-sex-change' territory. Especially with that final glimpse of the photo, showing that it was Dom who picked up the letter. For me, one of the biggest clues that supports this theory is when after recovering the bones at the end of last season, Red & Dembe went to pick up Dom before throwing the bones into a metal trashcan and burning them. I mean of all people, why would the imposter go to Katarina's father, to burn the bones of the real Reddington, unless there is some deep connection between Dom and fake-Red? The notion that Red is Rostova goes a long ways towards explaining their somewhat strained relationship.