Hi Nick,
My opinion is much the same as that from Paul and Neil.
Plagiarism of design features among the Victorian glass houses seems to have been rife, and agents, wholesalers and retailers of glassware certainly commissioned glass manufacturers to produce large quantities of glassware to their own specific design briefs, though no firm evidence has survived.
I have a substantial collection of reference photos of Victorian press-moulded or mould- blown glass items whose attribution is either unknown, unfounded, dubious or erroneous. On auction sites, many items with a Greek key design are attributed to ‘Molineaux Webb’, marbled or slag glass is usually ‘Davidson’ or ‘Sowerby’, and opalescent glass tends to be ‘Davidson’ or ‘Pearline’.
I think the generic term much beloved by auction houses and antique dealers is ‘in the style of…’, a coverall for much ignorance without admission of liablility, but without definitive indicators such as design registry lozenges/numbers or catalogue illustrations of unregistered designs for direct comparison then a positive attribution is difficult or impossible, and likely to remain so in most cases.
Fred.