When the Firearm Owners Protection Act was passed on May 19, 1986 two classes of dealer samples were created. Those owned by gun shops before the passing of the law, i.e. pre-1986 dealer samples, and those purchased by gun shops after the passing of the law, i.e. post-1986 dealer samples.
In summary, if you want to buy a machine gun on a Form 4, you will have to avoid pre and post dealer samples. These firearms are only available to a FFL with a class III license.