very nice Mike, good find for a tenner.
As you say, this particular double-dipping leaves the visible ring at the lower part of the neck where the second layer of glass come up to - often called half-post or German half-post (where it's alleged to have been first used). Suggestion seems to be that on the earliest of examples the second layer only came half way up the bottle, but never seen one like that. When I first saw this effect I thought the neck was inserted,
but it certainly isn't.As a production method it has its origins somewhere on the Continent of Europe around 1720/30, and was still being used in the States on a type of bottle they call a Pitkin Flask in the 1830s, but I think it had ceased being made in Europe before then.
Not to be confused with ordinary dip mould production (single gather). The date you suggest is as good as anything.
No idea as to the Spanish suggestion - when you say W/Fs, where in particular is that information from? Can't see a reference in McConnell for that origin - but then so much of C18 glass innovation comes from Bohemia/Germany that other origins may get overshadowed.
Meant to say............ a cautionary note - there's been some problems in recent years with repro half-post work, so be careful if you're paying big bucks.. but not of course if you're only paying a tenner