I need to clean the typeslugs a bit, especially the "e", but I am too excited to show this off. Corona made an extra key to clip onto the Corona 3 to allow for a full row for the touch typists who type by touch. This allows the hands to feel the home row completely and not shift over. It is, however, quite rare. There is an element of luck in getting one as few are ever listed. I saw one once on eBay for over $100. I wanted it, but couldn't justify that kind of cost. But, like most of us who have them I suspect, I found mine clipped onto a Corona 3. The Corona 3 did not interest me although it is in pretty good cosmetic condition (and it is missing spool covers and although paper feeds and the carriage seems to work, it needs some work at least some cleaning to free a few of the keys) so that is for sale for a little bit to help reduce the expense.
It clips on, and is very simple, however, I fear that manipulating it too much on the frame and removing it frequently could scratch the typewriter, so I was not too concerned with getting it perfectly lined up. However, it works for typing, because that pinky doesn't do anything anyway.
It is very slightly smaller than the keys on the older Corona 3. I put it onthe Standard Folding Typewriter.
The only other usability issue is that the keyboard is small, and it is still a double shift (which interrupts typing anything but the letters), but it is quite a wonderful portable typewriter and worthy of praise. Despite being over 100 years old, it is still quite useful.
In other news, I found that the scale under the platen has a plate with a window and pointer indicating the precise location of the carriage. Putting it at four inches will allow one to easily center the carriage for the case. I just figured that out after all these months...aren't I clever?