The 'Test Cock and Locking Bolt' is the correct terminology fella's and what you are referring to as the 'Thetis' clip was actually called the Safety Swing Bolt.
The test cock was pulled half way up and the reamer was used. If tested dry you would pull it all the way and the locking bolt will move out of the way of the tube locking ring. You would then open the tube by turning the ring. The safety swing bolt would be loosened but could not swing aside as there was a collar on the bolt which meant you couldn't fully open the door until you had cracked the seal. If ,of course, the tube was flooded you would know by now and shut the rear door. 8)