Section 602 makes holding an un-authorised sign at a Democratic or Republican National Convention, a Presidential, VP, or Candidate appearance, and any other event designated by the Secret Service as a "national special security event". a felony punishable by a year imprisonment. A not much more farfetched interpretation would have made felons of the entire Wisconsin Delegation to the 1968 Democratic Convention when Mayor Daley ruled them out of order for moving to adjourn the Convention and reconvene outside Daley's bailiwick.
Section 603 makes a seperate offence of entering the Convention (or other designated event) with forged credentials, possessing such, or even perhaps the time-honored tradition of passing ones' entry pass to a friend.
After some digging, I discover that this refers to the Patriot Act revisions installed by the Conference Committee. Their report, with their revisions, here.
"But," you say, "there is no way such a flagrant violation of our First Amendment rights will survive the Supreme Court. Those strict constructionists will never stand for it."
Keep telling yourself that, and realize that all that stands between the GOP and the First Amendment is the conservative*-majority Supreme Court.
*Meaning neo-conservative, meaning "Bush fluffer".