I saw this thread when it first started and frankly decided not to make any comment when the first four posts were all made by the original questioner. This left me realing with the number of questions asked and the complexity that had been created. The thread has gone on and only seemed to get worse
A question from me. Why on earth are you using a Harrods retail catalogue in oder to help work out the problem? The name they use is their own creation referring to items that they have bought on a wholesale basis from what could even be
more than one English maker. This means that these items would have no references on them that would identify the maker. The same happened at other companies, another being the London shop Liberty, who retailed for instance Thomas Webb items as 'English Glass'. Therefore it is useless as a primary source of identification and is likely to just confuse you - unless of course you have actually identified which company supplied those items?
Another query, why can't two or more manufacturers use the same (similar) colour in producing similar items, in this case what you are referring to as 'trailed' pieces? It happens throughout glass making history. Certainly manufacturers occasionally made ranges for tens of years, not just a couple of years after their design. If soething was commercially successful they would continue to produce it, if not it would likely just dissappear from the catalogue pages.
I'm astonished that you have given Miller's guides so much credance assuming that it is they who actually research the photographs that they use. In my experience through Millers direct use of my items, (and via other dealers) I can tell you that they rely upon the dealer, or the auctioneer who supplies the image to supply the information.
Millers come to the dealer (or collector) and take the photographs, which they then own and have copyright to. The owner of the item supplies a brief description, its designer, and/or manufacturer and of course the value. The former can be
inaccurate and the latter can be
manipulated according to any agenda that might be in the head of the items owner. Therefore, whilst a useful yearly publication, there can be
built in problems. This also can occur on the website, which on the odd occasion I have had cause to draw attention to an inaccuracy.
The 'mushroom' posy simply refers to a shape and is not dependant on size.
To be honest, although you are likely to begin to get a handle on this subject through looking at things in publications and on the Internet, the only way that you can really get the differences sorted out is to go digging in the archives of the relavant companies. Not such a helpful comment I know, but probably realistic I'm afraid
I find buying and owning identifiable pieces is the best thing to do. Live with them and the differences and similarities begin to emerge. Of course this can be expensive, but that is the nature of learning IMHO. Largely speaking, research ain't cheap.