FlightCaster.com shows that the plane making up DL300 is DL147 DCA/ATL
http://flightcaster.com/show/271407310
Neither one really adds up at the moment, because, DL1012 is slated to arrive at A26, DL147 is slated to arrive at T1, but DL 300 is slated to depart from A2. Of course, gates can change in a heartbeat!