I think this was later found to be incorrect. Supporters of the official theory then used it to discredit ALL of the anomalies in the investigation (even though they had nothing to do with Foster's left or right handedness).
There are plenty of other reasons, though, to conclude that the gun was merely a "drop gun" placed there after the death in a crude attempt to fake a "suicide." No fingerprints on the outside surface of the gun, for example.