PRETTY MUCH EVERYONE is familiar with the Corona Four; it's a smallish, four-bank single-shift portable introduced very shortly before Corona Typewriter Company was bought by, and merged with, L. C. Smith & Bros. to form L. C. Smith & Corona Typewriters, Inc.  An early example made by Corona Typewriter Co. and in my collection is seen at left; the front logo is blown up larger below.
We're also, of course, familiar with the fact that these enjoyable machines appeared in a variety of DuPont paint schemes, with many variations (like my red one, with black inserts or offsets) and that the Corona Four was made all the way through about 1941.
Now, there were a number of variations on the Corona Four, which we properly label as options, or else as special sub-models -- there was a Professional model, for example.

One less-remarked variation that actually led to a wholly different model, and one that is only rarely commented upon in collector circles is the
Corona Junior.  This was essentially the same machine as the Corona Four, but was contained in a totally different, much more squared-off body.  It was also almost totally devoid of extras - it had no margin release, no back-space, and so on.  On the left we see a scan of the diagram contained in the Corona Junior instructions.  These are pretty unusual machines, although not unfindable; they were introduced about 1934, it's thought.
We now have in our hands a further, much less identifiable machine.  This is a Corona Four -- it's clearly labeled as such -- but it's contained in what we have known as the Corona Junior's body.  I have seen perhaps two of these before, but not more.

This machine's serial number is as unlisted as was the serial number of my Corona Junior when I acquired it; the Corona Four's serial number is 1F 3078.  Luckily, Jay Respler has the official Smith-Corona serial number records (not reproduced in books, or on the web, in entirety) and dates this machine at 1934.
Corona Junior 1J 2070  /  Corona Four 1F 3078

It was natural to immediately compare the Corona Junior and this Corona Four, both seen here.  This exercise revealed that this newly-found Corona Four has pretty much exactly the same features as the common Corona Four.  Compared with the Corona Junior, this Corona Four adds paper holder fingers on the carriage and a left-end page guide; margin release key; back space key; three position ribbon selector; a right-end margin warning bell; and right-end margin stop.  Indeed, it has all the differences you'd find between ANY Corona Four and the Corona Junior.
At right, Corona Junior s/n 1J 2070 for comparison.  Note that the Four has two more keys on the right side of the keyboard, for margin release and back-space.  The ribbon selector on the Four is just above the shift-lock key.  Note also the open ribbon spool covers on the Junior, compared with the completely enclosed ones on the Four.

Other than these differences in appurtenances and features the machines are functionally indentical.  The body, as you can see, is the same as well. 

One is left to wonder why this particular variant has gone unremarked for so long -- how rare is it?  We're also left to wonder if this isn't a hint that, during the depression, only one body was used for these to save costs.  Or, when the depression ended, did the squared-off Four simply use up the supply of unused Junior bodies and parts?
The Junior, and this sub-variant of the Four, are interesting for one further reason; they mark a deliberate change from the old-style appearance of Corona portables toward a new, boxy shape that later mimics the L.C. Smith standard machines. 

Whatever the story behind this variation in the Four, it does give Corona collectors something new and different to seek out and research.