threads almost always reappear to those who have contributed posts, and looking at this one again I'll add a little more that possibly I should have posted two and a half years back. This Registration doesn't appear in Ray Slack's book, which presumably is where where I looked - why I didn't check Thompson seems now a big oversight. Also, looking again at the pix I'd suggest very strongly that this was decorated by means of sand blasting - the image looks very stencilly and grainy - and don't I recall that Sowerby did a variety of tumblers and drinking glasses that were decorated by the same means, around this time?
Certainly the decoration wasn't created by needle acid etching which has a very different finished look. I doubt that I shall visit Kew again, and presumably in my absence no one has made any efforts in that direction - shame.