Having a (current) copy of the law with you is not a bad idea
I have a friend who has a registered antique motorcycle. Under our law, he is not required to wear a helmet to ride it. Now, he is only supposed to ride it to and from antique motorcycle shows (which is generally what he does). And he carries a copy of the relevant law with him when he rides.
He still gets stopped. He still gets tickets. He tells me that showing the cops the law gets him out of about half of the tickets. The other half just write the ticket, and tell him to fight it in court. He does go to court on those tickets, and has had every one dismissed.
Street cops are trained on the law, but knowing all the minor specifics of every law on the books is hardly something one can expect from all officers. They are, after all, human. How you go about explaining to the officer that you believe they are in error and you are within the law is a exercise in tact, and should only be attempted in a non-confrontational stable situation.
Good Luck.