S&W issued a directive on December 22, 1959 to change the threads on the extractor rod from right-hand to left-hand. As I understand it, this was mainly a result of the extractor rod loosening during recoil generated by the 44 Magnum, but it applied to all models. However, it took the factory a couple of years to implement this change and all of the -1 models I have encountered (not many) were shipped in 1962. Again, this is based on my experience, and there may be exceptions as there are with most things S&W

. Over the years, the 27-1 seems to be the most prevalent, that may be a result of more of this model being made in 1962 or that collectors just report finding them more often. Needless to say, the -1s are scarce and this applies to all models. All -1s I have seen (except for the 29-1) have a four-screw frame and and extractor rod with a left-hand thread as evidenced by a small relief cut behind the knurled tip and and stamped MOD. XX-1 (fill the the model number) in the yoke cut. Often the number 1 is larger than the model number and appears to have been stamped by hand. The cylinder of these guns also has an L stamped on the rear surface indicating it has a left-hand thread.
The Model 29-1 is an exception, but I have no idea why. Most of these have three-screw frames and extractor rods with the old style or right-hand thread. Two in the hands of collectors are correct for 29-1s, except there is no L on the rear surface of the cylinder. I am sure this all relates to S&W using up old parts and creating "hybrid" guns if you will. The directive to implement the new cylinder stop, thereby eliminating the cylinder stop plunger screw that resulted in the three-screw frame, was issued on November 28, 1961 and implemented within a year or so. It is interesting to note that some of the early 29-2s have cylinders with an L stamped on their rear surface and were perhaps intended to be used with the 29-1s, but never were.
A collector could make "a career" out of collecting a studying the -1 models. I have studied the 29-1s, but not the other models which seem to be "normal" in how the engineering changes were implemented for them.
Bill