Never seen one before & considering it's a fitting that's never intended to be on show I think you might have to get a bit inventive to sort it.
You could try this http://www.stevensonplumbing.co.uk/chrome-hex-bush-3-4-x-1-2-inch.html
and get the angle grinder out to shorten it to act as a backnut.
Thanks everyone for taking the time to post, they're rare indeed.
It's to seal a running thread, if you don't know what that is - please don't ask.
Anyway, I ended up with a stainless steel one, polished it up with a pair of Olive's knickers, the snail trail made a remarkable job by the way!! That science for you!!
Job done.....
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