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I could see that line being a burn mark from escaping gas. Perhaps as stated earlier from a ruptured case.
Interestingly, I was looking at a H&H shotgun the other day and it had a circular groove around each pin with a straight radial groove extending laterally to the outer edge of each face. This was apparently in an effort to direct any gasses from a burst primer or case to the outside. It does not have bushed primers of course. Although the shotgun was presumably built this way, it is suspiciously similar to how this rifle has been eroded. Perhaps this problem had been observed before and the solution was to offer the gases an escape route which replicates the direction it wants to take in the event of a shell base failure.
Just wondering outloud...

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