yes, I've seen the reproductions of the Richardson mark on the Great Glass site, and I wouldn't presume to criticise that information, but some additional thoughts relating to such features, might be worth remembering.
There is perhaps justification for some leeway when attempting to use backstamps for dating - I think Bernard often mentioned we should be cautious in this matter - there are known pieces of marked Richardson glass that certainly slip into the early 1930's, although have to say that I'm not aware of a start date for their Union Flag mark - would be very interested to know if anyone has that info.
In Roger Dodsworth's (editor) 'BRITISH GLASS between the wars', he writes...........
""Glass Manufacturers tended to have a cavalier attitude to the marking of glass. Often two trades names in quite different styles would be in use simultaneously, and no records would be kept of when new marks were introduced and for how long they we used. Marks can therefore provide a rough guide to the age of a piece, but cannot be used for accurate dating.""
Some of the Stuart marks for example are well known for having been used over a long period of time.
However, far better to have a mark than not at all - scores on two levels - at least you know who made the thing, and it does give a rough indication of when.
Coming back to this wine, and having looked at a variety of pieces in C.H., it's quite possible that date wise it may well be later even that the Edwardian we have suggested............. as we've said umpteen times, such unmarked pieces are notoriously difficult to pin an age on, although your comments about the sharpness of cutting would suggest pre 1920 ish.