The first swivels were made up in the small machine shop at Holland and Holland by ladies called up for war work. The picture shows the stages of production.
They were made up of 3 pieces to drawing D5(E)5848/189 especially for the No4T. There were trays of complete and incomplete, partially made swivels at the old factory in the early 80's. Some were completely plain and some had a VERY small S51 logo stamped into the top chamfered edge of the rotating ferrule. Others had a simple WD arrow stamped into the head of the screw while others had an equally small S51 stamped into the head in place of the WD arrow.
Alas, all gone now