Prior to 1927 mainly law enforcement officers had Police Positive Specials modified by cutting the barrels down to 2-3" for pocket carry. In '27 to '33 the frame was basically a large frame Police Positive square butt and 2" barrel was labeled Detective Special and so the "factory" manufactured DS was born. '34 to '47 the grip frame was modified to a smaller butt with the front and rear of the bottom of the corners rounded for easier carry. That butt profile existed until '95 when production stopped. All DS's are steel with blue, nickle, or "Colt Guard" (rare) finishes.
The Cobra was introduced in 1950 and is an alloy frame finished in blue or nickle with a steel barrel and cyl.. It was produced till the end along with the DS.
The Agent was introed in '55 and ran till '61 as the first "short butt" model, again in Alloy with barrel and cyl in steel. I think it only came in blue and was designed to compete with J Frame Smiths.
The DS, Cobra, and Agent all got the short butt from '66 on. Again I "believe" the Agent was considered an economy model.
Towards the end of the D frame production turned to stainless steel and were called SFVI (small frame 6) and then DSII with same DS size and profile but totally different internals.
Within the parameters of all these guns are different available calibers, barrel lengths, barrel shrouds, and add on hammer shrouds.'
The Cobra was the 1st in the line of Colt "snake" guns. Cobra, Python, Diamondback, Anaconda, Boa, King Cobra, Viper, and Garter Snake :=)