hi - pix of the marks are just about o.k. - hope you won't object to some criticism - said in the most polite way, of course, and quality might be improved a little with the following suggestions

Plain and simple backgrounds, without fingers or other un-necessary distractions, are best............ for clear glass you might try using a charcoal or dark grey colour - for my money these darker colours are far better than plain white.
Try a dedicated light source that's fairly close, and if possible angle your camera shot to capture the light bouncing off the glass (area) you want to photograph. The point of doing this, and moving the camera to avoid massive glare, is to get the reflected light to capture small areas such as backstamps etc.
I think what in effect we're trying to do is capture the difference in light emission from the etched mark part of the glass, and the other plain shiny parts of the foot - and in the process get the light itself to outline the mark more clearly. Some folk use talc rubbed on the mark, some use carbon paper or similar. Otherwise marks can be notoriously difficult to highlight and capture, from the point of trying to show on the pc screen for others to see.
Another wrinkle that came via a lady in Canada, is to place a loupe/lens between the camera and subject - in effect you're then photographing an enlarged image - it needs care and practise, but the end results can be far better than simply trying to get nearer with a close up setting. It really does work, and is quite amazing when done well.
I think other more knowledgeable folk here have previously commented that' some care is needed to interpret dates for these backstamps - they seem to have had perhaps some overlap in use.
See your point about the difference in cutting of the fern - not that this is real cutting I suspect - more akin to engraving, since the lines are very shallow, but nonetheless a difference there is. Unfortunately, there's nothing we can do about this now...... the die is cast etc., and if you don't like the later versions then you have the choice to leave them alone. I guess though it could be a problem if you're trying to complete sets of these shapes/patterns, and I sympathise when someone is buying on line only, and doesn't have the opportunity to inspect the goods prior to purchasing.
We had this long thread some time back about the -S- or S occurring on some cut pieces, and thought it might have been a seconds mark, but it has been shown to occur on pieces that were destined for a large mid C20 retail outlet by the name of John Stonier of Liverpool, U.K.
As far as I know, Stuart didn't in any way specifically mark pieces considered to be seconds - but I might get shot down in flames for saying that... there are folk here who know much more about some of the larger British factories than me. Don't know if you've yet come across any Beaconsfield or Woodchester with this upper case S.
Appreciate your disappointment with this tailing off re quality of cutting, but think you are just going to have to live with the problem and accept that it will take longer than anticipated to complete your sets.
Also don't forget to use the Board's 'search facility' - very good for reading up on some of the input from the clever people, and from which we all learn much. A much under-rated benefit of the GMB, which I'm sure a lot of folk simply don't appreciate.