Below is a screenshot of the video showing an ejected empty cartridge just after it bounced off of the rear bridge.

Based on what I can see in this video, I think that there is reason to believe that the slight indentations on the other empty cartridges besides CE 543 might have also been caused by an impact with the rear bridge. The reason that CE 543 didn’t have one of those marks (that some have assumed are some unexplained chamber marks) is that CE 543’s impact with the rear bridge was closer to the mouth of the cartridge.
So, it happened repeatedly but just with Oswald's rifle and on every one of the shells, 30+, that was produced while testing the rifle in time trials, also including both shells listed as CE 557 and the unfired cartridge, CE 141, ejected after the rifle was found on the 6th floor. As absolutely incredible as this proposed idea is, and to take the disbelief to a new level, the depth of the chamber mark is different depending on when in the firing sequence the chamber mark on the shell was produced and the shells landed on the bridge of the receiver in exactly the same spot on every shell casing, no variance. As preposterous as all of this is, CE 543 still has no chamber mark and a dented lip that cannot be explained, both deformities indicating it was not fired in the rifle.