In 1953-54 Colt introduced two new revolvers.
One was the Trooper, the other was the Colt 357 Model.
The Trooper was offered in .22LR and .38 Special. It was available in blue or nickel, 4" or 6" barrel and the customers choice of any combination of the Colt Target grips, Target hammer, Service grips and Service hammer.
The Trooper was Colt's last "E" frame revolver which had the firing pin mounted on the hammer.
The 357 was both the caliber and model name.
It looks exactly like the Trooper except for the name stamped on the barrel.
It was available in blue with a few in bright nickel late in production, and with 4" or 6" barrel.
Like the Trooper it could be had with the customers choice of any combination of hammer and grips.
The 357 was Colt's first "I" frame revolver which had the firing pin mounted inside the frame.
The same "I" frame was used to build the super premium Colt Python.
The 357 was Colt's premium holster revolver, with the Trooper intended for less expensive police sales.
Since most people wanting a cheaper gun were buying the Trooper, and those wanting the best buying the Python, in 1961 Colt discontinued the 357 after about 15,000 were made.
In 1961 Colt started building the Trooper on the "I" frame and discontinued the .22LR version. After that the Trooper was available in .38 Special or .357 Magnum.
In 1969 Colt discontinued most "E & I" frame revolvers, keeping only the Python in the line.
They introduced a new Trooper Mark III series which was a totally different design using a transfer bar safety/ignition system.
In the early 1980's Colt did an upgrade and made the Colt Trooper Mark V for several years.
Finally in the mid-1980's Colt changed the barrel to a full lug type and made it in stainless steel as the Colt King Cobra.
So, you have the "original" Trooper in .22LR and .38 Special, the post-1961 "I" frame original Trooper in .38 Special and .357, and the 357 Model as made from 1953 to 1961.
As above, the 357 Model was to have been Colt's premium quality holster revolver. It was better finished and tuned then the original Trooper.