Looks almost as someone left a cartridge in the chamber and put it away. Seen cartridges that had leaked oxidic gases from the powder and coroded the brass. If so it would be seen at the extractor as well and it could be that the rim only rests at the action on the lower part thats why it looks that way. Or perhaps a galvanic oxidation from some fancy solvent left on the cartridge/chamberbrush.
The "crack" could be some line/flaw in the metal that got visible after filing then soldered and flattened.

Anyway a picture on the breech would be nice.