Looks to me like they are two different knots. In the illustration dudley presented, the standing line goes through the loop of the leader. In the Castwell Knot, the standing line is outside the loop of the leader.

When I saw dudley's illustration this morning, I knew it was different than the Castwell Knot as I tie it, but I had not had a cup of coffee yet so I decided to come back for another look later in the day.

The "knotting effect" may be similar, but the way the two are tied are distinctly different and strike me as two different knots.

John