A very complicated issue:
Screw numbering and screw count relative to frame designations:
1st rule: The K and N frame screw numbers and screw count are different from I and J frames.
Post war the I & Chief Baby J frames were all 5 screws; 4 screw sideplate and the 5th, the TG screw, i.e. 5 screw frames. The trigger guard screw is the 5th screw because it was eliminated first on these frame sizes with the introduction of the Model of 1953. So all I and Js became 4 screw guns after 1953. The 4th screw at the top of the side plate was eliminated next. So a 4 screw I and J frame has all four sideplate screws and can also be referred to as a 4 screw frame. Same with the 3 screw sideplate/frame.
A 5 screw K or N frame has 4 screw sideplates and a "4th" screw in the TG. Fourth because unlike the I and J it was not the 1st removed, it was 2nd c. 1960. The top sideplate screw was the first to be removed therefore it is the 5th screw.
So a 4 screw K or N frame has only 3 sideplate screws and the 4th in front of the trigger guard; again, the top sideplate screw, 5th screw, being the one eliminated first.
2nd rule: Always count the screw under the right grip because it isn't covered by the grip on most pre war models and 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 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.
Exceptions:
1. Early K Airweight guns can be found with a "6th" screw or "bug" screw which is a lock screw for the top sideplate screw similar to premium shotguns.
Since I frame Airweights weren't introduced until after their Trigger Guard, the 5th screw, was already eliminated with the Model of 1953, their "bug" screw can be referred to as a 5th screw and are the only Mod 1953 "5 screw guns". (pic and link below)
2. The original K frames, .32-20 and .38 were 4 screw guns; all 4 sideplate screws but no trigger guard screw and became "5 screws" when that trigger guard screw was introduced on the 3rd Model M&P HEs (Models of 1905).
3. The new X frame 500 and 460s went back to 4 screws.
Note: I'm not aware of any sources that include the flat hammer spring tension screw in "the screw count" of frame designations. Personally I feel it only complicates an already confusing topic.