You forgot 2010 to make it AUTHORIZED. It's repeating again starting in 2011 - we're up to AUTH as of 2014. Go back to 1994 and you'll see they started with H for some reason, but finished the same word anyway. I might have forgotten what the E,H and A were a reference to , but this has come up before as 1buckie said.
@Craig will know.
And for all the newbies here, a poster that shows every vent variant in chronological order from 1952 through 2014.