If the Sears model number of the follow rest you have is 214.10100, it wasn't made by Atlas (I don't think that I have the Sears Contractor Code list to say who actually made it). Unless you have the original box or the person you bought it from said that they bought it from Sears, there is no way to distinguish any of the Atlas built accessories between sold by Atlas or sold by Sears. The only things that Atlas or Craftsman made by Atlas ever put model numbers on were the lathe, mill, shaper and drill press nameplates (plus on the headstock or wherever there would be either an Atlas or a Craftsman badge). If the one you have was made by Atlas, the main body casting should have part number M6-396 molded into it. The Atlas model number was M6-395. But unless during the 30's or 40's Atlas and Sears sold them in different colors and someone actually knows what the colors were, there is no way to tell who sold them except the original box or invoice, or a buyer's affedavit..
Or to put it all a different way, if you already have a 6" follow rest made by Atlas, just assume that it was sold by Atlas. Likely no one can prove otherwise.
Robert D