These are alternatively called the curse of the flea markets, or cabbages. There used to be one in every stall. It has always been assumed they were Italian, but last Queensday I saw a rather large Czech woman systematically collecting these, muttering they were from her native country. I have one example with a rough broken pontil, and a triangular ashtray with a neatly polished base - so still no attribution, but the confirmation that these are around; you have not scored an alien!