wish you luck with this project Ross - not going to be easy dating these backstamps/labels accurately - like other factories I get the impression there was much overlap in the period of use. Some of the Stuart registered designs and their corresponding Nos. were long lived, so no help there I don't think.
Regret I can't help specifically, but just wanted to add to your misery by commenting that there is at least one more backstamp variant that you appear not to have included.

I'm attaching pix of a pickle that has a backstamp produced with an italicised font, plus the Rd. No. 682593, which was first Registered on 2nd May 1921. As you can see, the pickle has been produced in 'Stratford Rings' pattern, and according to my books 1921 was the year of introduction of this particular pattern - apparently it was a pattern which remained in production for one of the longest known periods, ending life somewhere in the 1970's I believe.
Just to confuse matters further, my books suggest that the word Stuart (with large squiggley tail) was first Registered in 1924, and was first used on glass c. 1926.
So, is this pickle evidence that this italicised mark was used prior to 1926, do you think - and is therefore the first ever acid backstamp used by Stuart?
Despite having seen vast amounts of Stuart glass both with and without backstamps, this remains the only example of this particular style of backstamp that I've ever seen.
You'll notice that the mark doesn't include a possessive apostrophe, which makes the final 's' all the more cuirous.
Do you consider any of the text book information on dating these backstamps/marks to be reliable enough to use for dating?
Sorry if you've already answered this question, but do we know the whereabouts of the factory papers/archives?