Colt Trooopers and .357s
There is still a bit of confusion about the various Colts of this type. I don't have reference works at hand, so my dates are vague.
As dfariswheel stated, the ".357" model is a sleeper. It was essentially a four-inch Officers Model Special chambered for .357. Most were in four-inch, but there were some six inch versions. At the same time, Colt offered this revolver in .38 Special and named it the Trooper. At a later date, they combined the two and offered the Trooper in both .38 and .357, and then just discontinued production of the .38 Spl version.
Around this same era, Colt introduced the Python, and the Trooper became sort of their "second line" .357, akin to the relationship of the S&W pre-model 27 and the Highway Patrolman, pre-28. The .357 Trooper, except for the very early ones, were less finely finished, externally. Still a superb revolver, though.
The Mark III series came later on, around 1970, with entirely different lockwork, coil srings and such. They offered the same lockwork in - -
The Trooper MKIII, still a .357 with adjustable sights,
The Official Police MKIII, a fixed-sight .38 and
The Lawman MKIII, a fixed-sight .357.
There may have been one called the Metropolitan MKIII, a .38 with adjustable sights. Not sure of the latter, though.
Best,
Johnny