More info Steve. I've just got the instruction sheet from Jack (I forgot it before!) and it says the
sockets are tapered so the white ferrules are locked in position. Here's a ferrule pic (from Rimmers).
So I think the tapered holes will stop the single cable being pulled through the end wall of the box by the shuttle block. Also, the thickness of the end walls/tapered holes means that the cables are held firmly in line, which should eliminate the risk of them popping out of the shuttle block during operation.
I've borrowed this pic from Jack's guide.
A comparison pic of the flimsier MGR box is at post 9
here.