Excellent analysis Fred, we have the same point of view.
This loco is not a Bing model, that is sure. I always sought to Leeds but they didn't produced many gauge one models and if finding documentation for Leeds O gauge is easy through the work of David Peacok it is harder for gauge one. I also sought to Jubb, unfortunately i do not have more catalog than you...
It is also possible that it could be a homemade model so I think i will never know more about it.
All my best wishes, Daniel