The only sure way to identify this is to get the info from the factory registers held by the REOC. Having said that, I think this will turn out to be a '56 model year Tomahawk which left the factory in December 1955, or possibly in the following January. The new late-type frame without the cast swingarm mounts, first seen on the '55 model year Trailblazer, was standard on the Tomahawk from November '55, by which time these would be to '56 model specification.
ERC is correct in thinking the '56 model had an SR2 mag, with alternator charging, but the engine would still be the older type long stroke 500 twin. I think the later short stroke engine was first used for the '58 model year, though it must be remembered these would probably have commenced manufacture in the latter part of '57. These crossover production schedules cause confusion over dates, sometimes!