Did the -400 have that vent? I thought it was just the classics...
My understanding is that it was originally the tube for mounting the sextant on the early models and it was “repurposed” as a smoke vent.
Die hard tab hounds used to crack the smoke vent in flight, and wedge the T handle with a ten pence piece, so that any cigarette smoke on the flight deck went out of the vent, and didn't filter back to the forward galley.
I remember a particular Captain on a Gatwick-Houston trip in 1998 doing almost exactly that! The only difference was that he used a £1 coin. Typical BA Captain; I’m surprised he didn’t use a £50 note to light up........