I would like to know when Mapplethorpe got around to photographing a coin. Nowhere, but here on eBay, have I ever seen this piece, and I've seen two pieces (a hard-boiled egg balanced on a pile of salt and a red tomato, shot from above, on a black dinner plate against a hunter green background) not subsequently published elsewhere.
I think if they found used toilet paper (should he have even used it!) they'd put that up for auction as well.
I highly doubt this is an authentic Mapplethorpe.