The difference between the Agent and Cobra depends entirely on the age.
When the Cobra and Agent were first issued in the 1950's the Cobra had the old full profile grip frame, and the Agent had a "stubby" grip frame.
Sometime in the mid-1960's, in order to simplify production ALL the small Colt "D" frame revolvers were changed to the "Stubby" frame design.
After this point, the only difference between the Agent and Cobra other than the barrel markings was, the Agent had shorter wood grips, and the Cobra had longer wood grips that overlapped the bottom of the butt.
The Finish of the Cobra and Agent was always Colt's usual first class, except for the short time during the big strike when Colt issued a series of un-polished revolvers.
When the strike was on, Colt didn't have enough qualified polishers to keep full production on every model, so they issued a limited time series of un-polished, flat black revolvers.
The Commando, also known as the Commando Special, was the black version Detective Special.
For some reason, Colt didn't give different names to the black Agent and Cobra.
As long as you don't buy one of these un-polished "Strike" guns, any Cobra or Agent will be up to Colt's usual standards of a shiny, well done finish.
Agent's and Cobra's made prior to the 1972 models with the heavy, shrouded barrels, were NOT factory authorized for ANY +P ammunition.
The post-72 shrouded version were rated for around 1500 rounds of +P.
In fact, many people did and do, shoot standard .38 Special for practice, and load up the +P ammo for "business' even in the older non-rated models.