Interesting thoughts!Some rambling thoughts. I don't know which criteria was used by Appleby to classify the entire Schick-Eversharp-Warner Lambert production.
Looking at the timetable i see three main classes of Injectors:
1) The Schick Repeating Razor (1926-1940). Apart from the fact that it uses injector blades I wouldn't even list it together with the later Schick injector razors even if it was the only designed by Jacob Schick himself.
2) Pre-post war Schick Injector razors (1934-1955). The one with the "art deco" head. Designed by Octavious V. Rodrigues.
Here Appleby made four distinct "types" (D-E-F-G) mainly based on the shape and the material of the handles.
Now, since 1939, how many zillions of different Gillette Techs have been made? Brass, bakelite, aluminum, plastic. Also so many different heads. But we just call them Techs, right?
In my opinion, the D-E-F-G distinction is too finicky.
There have been just a "Type D" razor. Same head as the early "Type E", it only differed for the scissor-like split metal handle. Only produced in 1934-1935. The same can be said about the "Type F". A special limited Christmas edition produced only for a few months. Same head geometry, same striations on the guard bar and same shave as a "Type E3/4/5".
Now with the "Types E" and "G" injector razors: the former had a bakelite handle and the latter a cheaper plastic one, pretty much the same razor except for some minor modifications through the years.
3) The "New" Schick Injector razors (1954-2001). Pretty much the same head for almost 50 years. Designed by Karl Leopold Kuhnl.
Here again Appleby made up six distinct "types" (Type I/J/K/L/N/O models) differentiating them by the shape of the handle. I would have rather differentiated them as:
1) Regular injector razors (same head for I/J/K/L/N/O models)
2) Hydro-magics (I/J/K/L "Types" includes at least a model with the hydro-magic lever)
My two cents on the different efficiency of the Schick injectors through the years.
It has been said and proved that the "Type E" is the more efficient/aggressive of the injectors and the following models were milder and milder with the passing of time until they reached the bottom level of efficiency with the "Type N".
My speculation is, as the injector blades became better and sharper, they tuned the level of efficiency of the razors accordingly. The main selling point of the Schick marketing was "Shave your whiskers... not your face" and they were known to give a smoother and faster ("half the time") shave than the competition.
Imagine the poor every-day shaver in the 70's using a Krona or a Super-Platinium blade in a Schick razor as aggressive as a "Type E". Aaah, the blood!