Paul — Davidson certainly made one or more specials in jade that didn't appear in their known trade catalogues.
One was the Fostoria
American pattern cube sugar / preserve, positively identified as Davidson by the inscription, and also made in flint. We still don't know whether this was a Fostoria mould with an inscription added by Davidson or a very close copy made by Davidson's mouldmaker. Hopefully in the not too distant future one will make its way over the pond for detailed comparison. Various scenarios are possible. My favoured one is that this was a special for a catering retailer who could see both cost savings and improvements in hygiene over the dreadful sticky bowl of sugar on the caff's counter.
Another came up on the GMB, culminating in this comment by Chris:
... I was wondering if there have been anymore thoughts about when Sowerby made jade? I picked up a jade T2505 9" salad bowl at Swinderby yesterday.
I have observed that this jade is like Davidson jade - it does not fluoresce under UV light. ...
I don't know why Chris seemed to disregard the possibility that Sowerby sent the mould over to Davidson for a short trial run in jade.
So, I can't see how Davidson can be eliminated, although I agree it's unlikely.
Bernard C.

ps — Paul, do you want me to bring my spare jade 283 5" Powder Jar lid tomorrow? Overall diameter 131mm, outside of locating ridge diameter 108mm. Please ask as it might not be on display.