pleased to say the V. & A. have now replied - as below. I wouldn't presume my knowledge to be on a par with such luminaries as Raymond Slack, Barbara Morris or Rebbeca Wallis, so will refrain from commenting further, other than to say that a definitive conclusion appears no nearer, and naturally, I've thanked Rebbeca for her time and trouble.
It does appear that presently we are only able to say that this design is likely Ed. Moore, and I'd still be very interested to know if Angus McDonald's book was published, and if his conclusions were any more substantial than the V. & A.
It's unfortunate that we shall never know the circumstances surrounding Barbara Morris' acceptance of the Prague museum Sowerby attribution.
Not that it in any way substantiates anything, but just thought it worth mentioning that in view of the fact that Sowerby's first Registration wasn't until February 1872, then it could be argued this could explain why all of the recorded examples of the Swan Vase lack a lozenge - just supposing this was a Sowerby design.
On the other hand Sowerby's invention of Vitro-Porcelain didn't occur until 1877, and I'm assuming these Swan Vases are all Vitro-Porcelain - apart from the Jet examples - is that correct Fred???

""Thank you for your enquiry. The vase you refer to in the V&A is currently attributed to Edward Moore based on notes we have on file.
Former V&A curator Barbara Morris and J. Thompson noted the attribution but give no reference. The note in the acquisition register is dated 1989, after Morris' 1978 book Victorian Table Glass and Ornaments where she refers to the Museum of Decorative Arts, Prague, as having bought this type / design directly from Sowerby in 1868. Raymond Slack states in English Pressed Glass: 1830-1900, pub 1987. p.108 that such colours as shown on this vase were attributed to Moore. I can't find the shape in any of the design books published for either factory - an example can be seen via this link
http://e-britain.co.uk/victorian_pressed_glass/pattern_books.htm Without further in-depth research, which I am unable to do at this time, I can only say the likely candidate is Moore. I would suggest contacting the South Shields Museum (Tyne and Wear Museums)which has a larger collection of Moore's glass as they may have more archive material to establish the source.
I hope this information is useful.
Kind regards,
Rebecca Wallis
Curator, Ceramics and Glass""
Sorry, meant to add............. assume this thread should now be moved to British.... thanks.