After having found that information, it's been years since any such thing has reappeared to expand upon or explain this breakdown of groups. We recently had a breakthrough.
I bought a brochure printed by the Pittsburgh Typewriter and Supply Company about 1918 (the brochure says "Established 1903" and describes the company having been "established 15 years") which lists a number of Underwood machines with both model numbers and letters.
The illustrations will be presented in the order of increasing complexity of features for each range of models, beginning with the regular No. 4. Next, the No. 5 which parallels the No. 4 but adds extra keys; and the No. 3 finally, which is the wide-carriage machine. I've never seen so many great cuts of Underwood standard machines in one brochure, covering such a wide variance in models. Underwood fans are sure to "go to town" with these codes and nuanced differences.
The original descriptions are included in the illustrations. Click to enlarge.
What we're pretty clearly seeing here is that this rebuilder (clearly stated as such) is offering a range of ages of Underwood typewriters of each model (3, 4 and 5) and that they have varied features --- with more appearing as time goes on. The machines are mixed up in the booklet, which is why I've clipped the scans and placed them in order. So that we can make sense of these things, here's a brief summation of the codes from this booklet.
No. 4-J 76 character machine
No. 4-L adds auto ribbon reverse
No. 4-P adds back spacer and two-color ribbon selector
No. 4-D adds lateral paper guide
No. 5-F 84 character machine
No. 5-R adds auto ribbon reverse
No. 5-M adds back spacer and two-color ribbon selector
No. 5-E adds lateral paper guide
No. 3-T has all features of 5-M but wide carriages
No. 3-K has all features of 5-E but wide carriages
We can try to zero in on the age range of the machines you've seen using other evidence. For example, we know they're all post-1900. Also, note that none of the machines you've just seen (except for the 3-K) has a shift lock key on the right side of the keyboard; they're thus all pre-1915 (or so) when that feature was added... again, except for the 3-K.
The OMEF serial list, looking at the No. 5 model, says that back space and bichrome ribbon began on that model at about serial 245,000, or in the 1908-1914 time range. Looking at tw-db.com we see that these features are listed on the No. 3 as having been added in 1909 - so that's a good solid guess for the No. 5 to have added the back space and two-color ribbon as well. This then places the 4-P, the 5-M and the 3-T starting in 1909.
The brochure, as noted, is from about 1918. Right now, it doesn't look as if it matches up properly with the groups from the rebuild manual - the rebuild manual's K groups are placed by serial well after the print date of this brochure. What we do now have though is a couple of solid pieces of evidence that, internally, Underwood was referring to changes in features by letters suffixed to the model number of the typewriters; it remains to be seen if we will ever be able to find a complete listing of such codes.
Let us know, in the comments, if you have anything to add to this code mystery!
5:20 PM 9/11/2013