They weren't. They only showed up as evidence after the trial had started and were taken from their evidence bag and examined by the judge and prosecution and defence in the courtroom. There is no record of them properly forensically examined, as far as I am aware.
It's irrelevant, though. They weren't declared - for reasons we still don't know - so it doesn't make any difference whether they matched the round that was discharged. The defence was given no opportunity to examine them which is inexcusable. Moreover, the manner in which they came to light suggests that they were intentionally withheld by the prosecution which is even worse than inexcusable, imv.
The problematic thing is, though, even if they were proved to have come from the same batch it makes no difference. It was AB's actions which were on trial which are irrelevant to how the gun came to be loaded.