There were at least two 50000 miles test cars. One was 514 and is now on display at Cultra. The other was D24 reg RIA7123, shown in many photos, but was destroyed by a fire apparently.
At least this is the accepted opinion on this from the guys at DOC who have researched this.
Of course I have never seen a pic of both cars together or of a reg plate of the second one. I assume the Cultra one doesn't have a plate.
Nick Sutton thought there might have been a third one also.