As we're having our lounge replastered, I'm taking the opportunity to
extend the ring and add a couple of extra double sockets in convenient
places. I've done this before and I'm quite happy with the wiring side
of things but, in the past, I've always used the plastic surface-mount
boxes to do this.

As I'm going to be adding the sockets between the time the old plaster
comes off and the new plaster goes on, I want to use steel knockout
boxes cut into the brickwork to give a flush finish matching the other
sockets in the room.

Never having used the steel boxes before, I understand that you are
supposed to put a grommet on the knockout to protect the cable.

Looking on the screwfix site they have 'open' and 'closed' 20mm PVC
grommets - the images look identical, so what's the difference and is
there an advantage in using one type over the other?

Many Thanks

Perry