I ordered one of these from an Amazon Marketplace seller, it was advertised as genuine and the vendor insisted it was "genuine Nikon" when questioned.
A number of aspects raised my suspicion when it arrived it so I consulted Nikon-Europe.
Nikon pronounced it counterfeit. The visible item they picked out was that the "Nikon" logo moulded into the upper side of the battery was different from the genuine one.
when varying the viewing angle angle, the hologram behaves differently from two other genuine Nikon batteries in my possession,
weight is 72g (3 oz) against 88g (3.5 oz) printed in the Nikon information sheet,
four small nicks in the hologram are cut-outs on the genuine item, on the counterfeit version they are slits,
squeeze the battery - the counterfeit flexes, the genuine one doesn't,
the box and data sheet accompanying the genuine item are of distinctly better quality than the counterfeit - with the latter, it is very difficult to fit both items into the box without it deforming.
After I sorted out a refund, I approached a second low-price Amazon Marketplace supplier and asked him to confirm provenance of his battery prior to me ordering it, the seller declined to comment.
Having trawled the web and looked at what established retail suppliers are charging, my feeling is that any advertiser currently asking for about 50 GBP, or less, is a risk not worth taking. One advertiser actually lists the weight as 72g, effectively confirming it as counterfeit.