these things do seem to have a special appeal for some people - about a dozen surfaced in the two separate sales of the Michael Parkington collection, back in 1997 and 1998 - so he was obviously quite keen, and had managed to collect what look to be most of the varieties of marbled glass colours, plus a pair of what were described by Christies as 'jet'.
Apart from the pair in jet, Christies used the word 'malachite' to describe all of the various marble colours - which we probably wouldn't do now, preferring to say marbled, although Christies did refer to the moulding as representing swans - which agrees with the books.
The auction house included the 'attributed to Sowerby & Co.' statement, but went on to say 'unknown maker, and gave the Barbara Morris reference quoted above.
I wouldn't dare comment on these things since I know nothing about them, but my interpretation of Ray Slack's comments are that he wasn't suggesting that all of the colours are wrong for Sowerby ....... he says "appear in many colours which are not associated with Sowerby".
I get the impression that Sowerby made a variety of colours in marble glass (malachite), plus of course white vitro-porcelain, and jet.
Regret I've not seen a single example - but agree they're appealing ...... is the connection along the same lines as Leda and the swan?
The Victorians were big on these Greek classical mythology things .. or perhaps it's just a vase with three swans