Not necessarily. The evidence I've heard about seems to indicate that the DA has to wage the assertion that the magazine is not pre-ban. Otherwise they wouldn't be calling up Glock, etc, to try to get them to rat on their customers. The "We think your mag is illegal" charge has to be backed up with something in order to get a jury (or a judge for that matter) to suck for it. "We think it's illegal but we don't really know" isn't exactly compelling evidence in a courtroom.
-Mike
Sorry, perhaps that was too strong. Of course the DA would have to prove it was post-ban, just like they would have to prove every other charge against you. But your Defense team would be equally motivated to prove that it wasn't post-ban. It doesn't change the amount of work by your team, and you start from the position of being charged with something that's not illegal (as SteelShooter points out).
Perhaps this happens more than I'd like to acknowledge.