I think all or most anarchists have adopted this sign as a sign of the 'rage against the machine', their desire to rise up and bring down their oppressive overseers just as V did. Before then IIRC it was a simple 'A' for Anarchy.

EDIT: Watching the video when he opens fire, you can see the chap is either extremely nervous or hasn't handled firearms before, he fires one and misses the man he was shooting at (thankfully), then has an accidental discharge into the floor...and then it sounds like a 'click', before he is then shot at from off camera and returns fire before falling to the ground.

EDIT EDIT: Kudos for Husfelt for trying to reason with the guy, and for Littleton for trying to disarm him. Both could have been shot, and it's a miracle that they weren't...particularly Husfelt. He sounds like a good chap, Husfelt, willing to stand up and offer himself as the man who 'signed the papers'. Both Husfelt and Littleton definitely deserve commendations.

Youtube gives a lot more perspective to it. Amazing the safety officer came in and went back out, not sure if it was the same one that shot him. Wife said she heard the officer was in the room the entire time and many saying he should have took action before.

http://www.youtube.com/watch?v=s5Rmr-X7B3c

Ginger had no brick but she DID have good aim. She took aim at the gun trying to get him to drop it.