Hi Richard,Excuse me for jumping in at this late stage, but I have been reading a new book Gothic Clocks to Lantern Clocks..Short-Duration Clocks & Rural Clocks 1480-1800 by John Robey. Chapter 5 covers English Lantern Clocks & Thirty Hour Clocks and describes in some detail a tall and slim rustic 'coffin' longcase clock with a movement converted from a Lantern Clock in Somerset c1680/1710. The case and converted movement look very similar to the one you are talking about here.
I appreciate your research, and would love to see that article. There are so many books to read, and some are very expensive, so I really appreciate it when people take time to look in their books to help me out.
I am sure there are a few converted lantern clock movements out there in longcase clocks, and usually you see the squares at the tops of the columns typical of lantern clocks, without which the spacing between the plates would be wrong. However the turned column birdcage movements I have found so far are regionally localised and cover quite a long period of time, and don't have the squares. I think the chances that most of these are lantern clock movements adapted into longcase clocks is exceedingly unlikely. However there are one or two that could fall into this category, and here is a link to an example described by Brian Loomes - but I think these are the exception.
Collecting Antique Clocks: A Sleeper Awakened, by Brian Loomes, writer and dealer
Maybe they felt they were perpetuating the lantern clock style by using turned columns - though given in the longcase the columns are hidden in most or maybe all of the early clocks, then the clock owner would hardly appreciate it.