It defiantly looks like brass transfer from the mouth of spent casings contacting the receiver, I've seen it on other rifles but not on my A5M. I would imagine based on my experience the brass transfer would come off with a little cleaner.
I'd cycle a spent case slowly and see how long the extractor is hanging onto the case, then also place a piece of black electrical tape on the rear of the receiver and cycle the action with the spent case quickly to see if it leave a mark on the tape.
I'd cycle a spent case slowly and see how long the extractor is hanging onto the case, then also place a piece of black electrical tape on the rear of the receiver and cycle the action with the spent case quickly to see if it leave a mark on the tape.