Jeremy, your engine was made 287 units before mine. It'll be interesting to see what your frame number is. I've got a sneaky feeling that the factory didn't go to any effort at all to coordinate frame & motor numbers. I'll be it was purely at random, which is why we can only "guesstimate" model years.
Wayne thought mine was a '58, but Bill Erickson thinks it's a '57 due to other bikes which he has knowledge of.
Jeremy, does your have the Phelon or Wico magneto (does it have a bunch of screws holding the cover on, or just 3 clips)?
Mike Sal