The "J" frame Trooper MKIII/Lawman series replaced the old Colt Trooper E/I frame guns, of which the Python is a version.
The Trooper MKIII was to be a good step below the high quality Python, and was intended for the police department and sportsman who wouldn't pay the high price of the Python. It was popular with a number of departments, including the U.S. Border Patrol, who bought it with the barrel marked Colt Border Patrol.
The "J" frame was to be a less costly to build service type revolver. It was one of the first modern transfer bar guns, which most revolvers since have copied. There were a number of "J" frame guns made, including the Trooper MK III, Lawman, Official Police MKIII, Metropolitian Police, Trooper MKIV, and Peacekeeper, among others.
The King Cobra is an updated "J" series revolver, which replaced all the others.
The Trooper MKIII was Colt's "cheaper" gun, who's finish makes most newer guns look poor by comparison. They are top of the line guns. Most people who have them really like them.
Barrels lengths on the Trooper MKIII's was 4, 6, and 8 inches.
Calibers were .38/.357, .22LR, and .22 Magnum.
Finishes were blue, bright nickel, and electroless nickel, also sold as the "Coltguard" finish.
The "J" frame guns have an action somewhat stronger than the older E/I frame Colts.
If you ever have the chance to get one, you won't be disappointed.