The only thing I can add - there are some categories of pressed pieces, such as tumblers, where trying to identify the glassworks is virtually impossible as the designs tended to be common and unremarkable. I guess you have to decide if piano insulators fall into that category or whether you think it's realistic to attribute them to a glassworks, when one finds an unregistered piece which matches a catalogue image.
To give an example of the problem, I have two early plates which appear in the Molineaux Webb catalogue - a Queen Victoria coronation plate, and a lacy Sandwich style plate. I would dearly like to be 100% sure my plates are by Molineaux Webb, but realistically, I know other glassworks were likely producing the same item, or with slight variations. I can prove Molineaux Webb used the pattern, but I can't prove nobody else did - and that's with a complex plate design.