I've always wondered about this. The story I recall hearing was that when the spares company they went to a filter company to see about having a brand new, modern, paper filter made for the machine. The engineers at the filter company were very amused at the cotton depth filter as supplier by Vincent, and were confident they could improve on it considerably. A few months later they supposedly called back, and somewhat sheepishly said that due to various oddities of the Vincent design, a paper filter would not be appropriate, and we would be better off sticking with the old hydraulic filter.
However, I don't recall hearing any details about just why the weren't appropriate, and since then paper filters have come into common use, with no apparent problems. Anyone recall what the objection was?
Oh, does anyone know whether there are differences between the paper filters the various vendors are selling for the twin, or are they all the same thing?