What determines what is "preban" is whether the rifle was assembled into an "assault weapon" configuration prior to 9-13-94. That means it must have had the ability to accept a detachabe mag and two or more of the prohibited features prior to that date to be a preban. For the most part, serial numbers can only definitely prove a weapon is a post ban, they cannot necessarily prove preban status. Also note that if charged, the burden of proof falls on you to prove it was assembled in such a manner prior to the ban. A manufacturer can tell you when a rifle or receiver left the factory, and in some cases they can tell in what configuration it left the factory. If it left the factory prior to 9-13-94 with 2 or more assault features, you're pretty safe. If, OTOH, it left the factory as a stripped receiver prior to the ban, you should get some other proof from the seller or person who possessed it on 9-13-94 to prove it had the necessary features on that date.
As to the DR200; I believe all came in the country after 1989, and are thus subject to the more stringent regulatory provisions under 18 USC 922(r) for imported weapons. Thus even though they might have come in slightly before 1994, no one was making US replacement parts at the time, so they are subject to the 1994 AWB (18 USC 922(v)) also.