The British method was - as might be expected - very different from the radial pressure measurement used on the continent.
I do not have detailed knowledge of it, but basically it measures the base force acting on the bolt face, using a well oiled case.
The original bolt is replaced with a pressure bolt so that the base force deforms a copper crusher within this device. It could be, the photo shows this UK crusher type. I have never seen one.
The advantage of this method is that you need no special pressure gun, and no drilled case. The disadvantage is that pressure figures obtained this way are very different from the CIP/SAAMI figures. British 9 mm is a case in point.
CIP/SAAMI copper crushers are little cylinders, diameter something like .28 in (several sizes exist), and have no central hole.