I'm quite sure I won't be allowed to post a link, but it's the other stamp forum that sometimes gets a mention here.
For what it's worth, and given my opinion is on the Australian board, I don't think it's genuine, but that's no guarantee I'm right

My comments verbatim:
"The more I think about this stamp, the less happy I am.
I examined quite a few double lined 5 watermarks (2d & 5d diadems, and 2d DLR) and none have the top even closely resembling the one shown here.
There's also two cancellations, one bold and one feint.
I'm far from the expert with regards to rebacking stamps, but could this be the case here?"
I saw it sell on
ebay. You could submit it for a certificate, but it's expensive and you'll be waiting a while. Until another double 5 watermark appears with a similar top, I'll remain hesitant.
As for getting it wrong, see my post below.