Just WHAT constitutes a Detective Special "Generation" depends on who you talk to.
The most widely accepted sequence is this:
First: The original model as made from 1927 to sometime in the 1960's.
This version had the long grip frame and the exposed ejector rod.
Second: This was the change to the short "stubby" grip frame. This was introduced in the mid-60's.
Third: This was the new heavy, shrouded barrel introduced in 1972-73.
Fourth: This was the last version, and was just the Third re-introduced in 1993.
Although this is the "generally accepted" sequence, some collectors have a problem with this, and you could make a good case for THIS sequence:
First: The REAL original DS with the long SQUARE butt as made from 1927 to 1933.
Second: The DS as made from 1933 to the late post-war 40's with the long ROUNDED butt.
Third: The post-war change to the new spring and plunger cylinder detent system, the ramped front sight, and the assigning of letter codes to the frame size. This was when the small Colt frame was first known as the "D" frame.
Fourth: The 60's change to the "stubby" grip frame.
"Somewhere" in here, the ejector rod was changed to a longer version that was just slightly shorter than the barrel.
Fifth: The 1972-73 heavy, shrouded barrel.
Sixth: If you get really technical the re-introduction of the DS in 1993 counts, since the model WAS discontinued, and Colt's that are discontinued, then re-introduced have always counted as a generation.
Seventh: The new 1995 Colt "SF" stainless guns including the SF-VI and the DS-II.
Eighth: The .357 Magnum Magnum Carry.
Most people DO NOT count the "SF" frame as a Detective Special generation, since it's an entirely different gun, not simply an altered "D" frame.
This would be like claiming the King Cobra is a generation of the original Official Police.
The WWII Commando was simply a war-time Official Police with a parkerized finish and plastic grips.
During the big Colt strike, Colt's polishers were out, and they didn't have enough qualified people to do all the polishing needed, so they assigned the people they did have to the premium guns like the Python and Government Model.
To enable them to produce guns, they introduced a series of their standard guns in an un-polished, flat black finish.
This series included:
The Peacekeeper, which was the Trooper Mark V.
The Commando, also known as the Commando Special, which was the Detective Special.
And black finished Agents and Cobras.
It's unknown why Colt didn't assign new names to the Agent and Cobra versions.
This has led to some confusion with the WWII parkerized Official Police known as the Commando.
So, you have your choice of what constitutes a Colt small frame Generation.
To avoid confusion, most people use the most widely accepted version which is the First, Second, Third, and Fourth.