Ah - I was going to offer to come round and lend you a cartridge to double check the problem if you lived within travelling distance but Sydney is just a tad out of my way
The SMMC4 is far superior to the orignal B&O equivalent MMC4 sound wise, not as refined though in the manufacturing but well worth the money. I also have a B&O MMC3 (which is the next step up from the MMC4) and the soundsmith cartridge out performs that too IMO. So The SMMC4 would be my recommendation although if you have money to burn then the sky's the limit with The Voice cartridge.
I've just check both of my tangential decks (6500 & 9000) and they are the same as yours re:- the black plastic disc and both of them play perfectly so this appears to be normal.
Chris.
The use of metaphors should be avoided like the plague. They're like a red rag to a bull to me.