The break in the forums at 1961 aligns with 3 screw gun production on K and N frames, and is a little after the use of engineering change dash #s began. That may be it, but that's a good question for our administrator, Lee Jarrett.
I can help you on the screw counts and dates however:
THE SHORT ANSWER:
Screw count is collector 'shorthand' to identify different vintages of otherwise same model guns MOSTLY for during the S&W rapid evolutionary period of the early post WW II years. It's based on the order of certain screws being eliminated by the factory as their model production evolved prior to and during the early factory Model dash number system. However the screw count has also evolved a bit as more Smith knowledge is learned and new applications have arisen.
SCREW DATE CHANGES:
• Eliminate mainspring tension screw, the 6th screw, as part of the I frame improvement to a coil mainspring, 1952.
• Eliminate front trigger guard screw, the 5th screw, as part of the New Model of 1953 I and J frames redesign.
• Eliminate upper sideplate screw change order 12/13/55 the 4th screw on I & J frames, on 9/7/55 the 5th screw on K frames, and on 9/7/56 the 5th screw on N frames.
• Eliminate front trigger guard screw, the 4th screw, from K and N frames, the dash 2 engineering change, 1961.
THE LONG ANSWER and probably more than you wanted to know:
Various gun collector terms/slang have surfaced over the years, some general but most specific to certain brands and models. The reason for these terms is usually to differentiate between and more specifically to identify different vintages and/or styles of similar models. Other examples are "pre model", "pre war", "post war", "baby Chiefs", etc., etc.
The miss-understanding and therefore improper use of several of these terms causes much consternation among many enthusiasts and forum users to say the least. Some will even argue about it rather than learn about it. There are no “rules” or “terms Police” for the use of these terms, but there is conventional wisdom which helps make their use more consistent and therefore more helpful.
1st understanding: The I and J frame screw numbers and screw counts are different from K and N frames.
2nd understanding: Always count the screw under the right grip because it isn't covered by the grip on early post war models up to around 1950. It’s a domed head screw when not covered by the original factory grips and a flat head screw when covered. In the early days of the Magna stocks, a relief dimple was drilled into the back side of the right stock panel to provide clearance for the round head screw before it was replaced with a flat head screw.
3rd understanding: There are exceptions as always with S&W firearms.
Screw numbering and screw count relative to frame size designations:
I frame collectors count the main spring strain screw as the 6th screw, which is like the K and N frames. But unlike them the strain screw on the I frame was deleted. All I frames dropped the 6th screw in the early 1950s, hence it is included in the screw count. And it's a helpful identifier between the early 50s I frame and the ‘Improved’ I frame.
Pre & Post war Transitional I frames were all 6 screws counting the strain screw, 4 screw side plates and the 5th, the TG screw. Baby Chief J frames were introduced as 5 screws with a coil main spring and no strain screw.
The I frame 6th screw was eliminated first with the Improved I frame c. 1952 (except the .22/32s which were not made on the Improved I frame and have no 5 screw version).
The trigger guard screw is the 5th screw because it was eliminated next on I and J frame sizes with the introduction of the Model of 1953 New I frame and New J frame respectively (.22/32s lost the 6th and 5th screws at the same time). So all I and J frames became 4 screw guns in 1953.
The 4th screw at the top of the side plate (called the "bug" screw at the factory) was eliminated next. So a 4 screw I and J frame has all four side plate screws and can also be referred to as a 4 screw frame. After the 4th screw (upper side plate screw or bug screw) was dropped they became 3 screw side plate/frame c. 1956, while the K & N frames didn't begin to transition to 3 screws until 1961, when the TG screw was finally eliminated.
K & N Frames: On K and N frame 5 screws, the upper side plate screw (bug screw) was the first to be deleted, therefore it is the 5th screw. So, unlike the I and J frames, the "4th" screw is in the TG. Fourth because it was the second screw dropped c. 1961.
So a 4 screw K or N frame has only 3 side plate screws and the 4th in front of the trigger guard; again, the top side plate screw, 5th screw, being the one eliminated first.
Exceptions:
1. 1st Model .32 HE, Model of 1896 with no trigger guard screw, and 4 or 5 side plate/frame screws thru out production, has two completely different 6 and 5 screw counts. The 6th screw being the top side plate screw.
Pictures of 1896 HE screws:
http://smith-wessonforum.com/s-w-hand-ejectors-1896-1961/340243-pictures-he.html
2. The original K frames, .38 and .32-20 were 4 screw guns; all 4 side plate screws but no trigger guard screw until the very late model of 1902, which introduced the 5th screw, in front of the trigger guard and are the scarce transition guns - about 10,000 of them. All became "5 screws" when that trigger guard screw was introduced on the 3rd Model M&P HEs (Models of 1905).
I hope this is helpful,
