Some ARC thing, came attached to a piece of pipe that I used to get my bonnet shut after the 30 went in.
The problem was, at idle, it was PUSHING air OUT the bov.. I can only put that down to the undersized turbo spooling from idle.. it wasn't making positive pressure but certainly positive airflow at idle! I mean, you would expect it to be a VACCUM leak at idle (i.e. sucking air IN).
So to stop it blowing air out at idle I had to tighten the bov pretty much as far as it would go.