While doing some reading on CE 543, the dented shell casing, I came across these two old threads on the Ed Forum. Tink Thompson participates on both (and in fact started one). The discussions get pretty technical, but I found them interesting
"An unfired cartridge?" (2007): https://educationforum.ipbhost.com/topic/9276-an-unfired-cartridge/
'The provenance of CE 543 and Gary Murr's recent report" (2009): https://educationforum.ipbhost.com/topic/14177-the-provenance-of-ce-543-and-gary-murr%E2%80%99s-recent-report/
They also serve as a somewhat depressing reminder of the quality of the discussions on forums such as this before the inmates took over the asylum. Now I return to the real world. 
Forget the dented lip. This is not about the dented lip or the mark from the magazine follower. Wrong indentation and mark.
Read the book. Pages 140-146 and the foot notes to the chapter on page 178. This is about the "chamber mark" as noted by the FBI, nothing else, CE 543 lacks the "chamber mark" the 30+ others do not.
Funny Josiah knew of a key piece to the puzzle back in the mid 60’s, puts it into print in his book Six Seconds in Dallas, but apparently does not understand the relevance and significance of it because he cannot weave it into a conspiracy. In 1967, he felt it proved there was a shot from somewhere else that LHO only fired twice. He must have given up on that theory. Josiah giving up on his theory in no way diminishes the importance of his observation. He is the only person to view all these shells that were fired in the rifle, compare them, and write about them.
The bottom line based on his observations, CE 543 is the only shell that lacks the indentation referred to as the “chamber mark” by the FBI, whereas the next 30+ shell casings including the unfired cartridge all exhibit to some degree the “chamber mark”. The “Chamber Mark” is not the dented lip. They are completely different, as is the mark from the magazine follower. READ THE BOOK.
