They would have been added based on the criteria that was used from May 1, so they would have all been semi-auto, sustained rapid fire, military tactical design capable of receiving a large-capacity magazine. They would also have been modern design, meaning post-World War II, and there would have been a prevalence or a market volume test in excess of 1,000 known firearms within the market at the time.
They would have met all three of those criteria in order to be added to the list of the 1,900 makes and models.