Pardon my ignorance, but I understood that the first number (6) indicated stainless finish, while first number 4 or 5 (eg., 459, 4006, 5906, etc.) indicated a blued finish throughout this series. Of course, there are always exceptions …
I also have a 4” 29-3 with similar hard chrome finish. Mine also has a very dull plastic front sight insert, as if it wasn’t removed before the application.
I have a 1937 Brazilian contract 1917 marked with small S&W logo on left side, re-importer stamp "IA CO SAC CA", lanyard ring, flat top strap, and square notch rear sight (Serial 203,xxx). I recall reading years ago here on the forum that heat treatment improved in the early twenties (over what...
I can second H Richard's reply. I had Cylinder & Slide shorten an extra barrel on a 1917 that had been mishandled by two earlier 'smiths'. They did an excellent job.