No sooner that I posted that last reply, I was looking through a 1927 Talking Machine News in search for something else when I found the attached ad for the XL. This makes no mention of the word 'Junior', but thinking about it, it does appear on the machine itself -- enlarge that previous photo I posted (and I realise it's not the best photo) and you can just see the words 'Junior Portable' under 'Decca'.
This ad also confirms that, at the time of its introduction, the soundbox, with its Duralumin diaphragm
was new and special to this model, though of course it might have spread to others later.