Well, I come to discover that the MPCORB.dat file on the observatory computer was dated January 2008 - pre-error-fix! In sorting through the April 2009 problem Don was having, I did remake my copy for simulation on my in-house computer, but I never copied that file out to the observatory.
I know this won't be a problem for me from now on. Case closed.
Just for fun I've attached an image showing the two FOVs mosaicked - on the right the predicted position of Concordia and on the left the actual position. North is up and East to the left.