Looks like a '56 to me. Does it have a brake light switch bracket just behind the motor (on the right lower bar)? The leather seat may not be original, or might have been a change over, as Simplex did a lot of that. If there is no brake light bracket, it's probably a very late '55 model.
Send me a personal note with some more info about you (real name, the state you live in, and a little more info about the bike (if the previous owner gave you any). I'll get you added to the registry. I'll be updating it very soon, since I got some more bike info during the Portland show.
Mike Sal