<p>The coupling (20) has a threaded fluid connection. It also has a threaded split clamping shell (22) which can be drawn into an external bush (24) with a longitudinal channel (33) between its ends (32,34). A stretching pin (26) is preferably connected to the external bush by a cross member (28). The pin acts with the split clamping shell so that the shell expands or contracts when it is fitted to the threaded fluid connection of a proofing body.</p>
