The last tool used to finish making the chamber, where the indentation is originating, is a reamer. Possibly it had become defective and left a little burr or something similar in the chamber.
I'm not familiar with the manufacturing of the carcano..... But usually the last tool used in finishing the chamber is a hone....
The hone removes any burrs or roughness in the chamber and polishes the walls of the chamber to allow the spent expanded cartridge to be easily extracted.
This is what you took out of this discussion. Did the Italians take the time to finely polish the chamber like these rifles were fine hunting rifles instead of just using a finishing reamer? The shell information are facts that help explain the assassination. CE 543 does not have a chamber mark. Every other cartridge or shell that was introduced into the chamber of the rifle has the chamber mark.
The only conspiracy left is why did the FBI not explain why the chamber mark was on CE 544, CE 545, CE 141, CE 557, and 30+ more shells, as noted by Josiah Thompson, but wasn't on CE 543. The WC Conclusion included the belief that LHO may have carried an empty shell in the rifle and ejected it before shooting. They understood this was a possibility but did not feel the need to explain the "chamber mark."
To add a pointless step to the manufacture of the rifles instead of using a finishing reamer is of no use or is of no real importance and in no way explains indentations on the side of the shells from the chamber. Apparently, they left the burr because it shows up on the shell casings. Especially since there really is such a thing as a finish reamer. Grinding a chamber, to remove a burr, wider and out of spec with a hone is counterproductive.