quote today from m ............... "but is Christine's here no 4 maybe?" ........... possibly m, but in the absence of corroborating information in the form of dimensions - details of which are not shown on the original factory catalogue page - then we can't be sure - there are probably at least two insulator designs with an octagonal profile - Christine's and your own example. I suspect that either yours or Christine's are possibly PV, but we may never know - understandably, mass produced utility items such as insulators were never going to be considered worthy of much detailed attention, and the majority I see are without any Rd. data.
Neil was asked for his opinion back in 2012 when Christine posted her insulator, and he gave the following comments............
"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.""
I've seen very few Registered designs in the Kew archives.