"A Pre-model is any named model that incorporates all design changes that immediately precede when the company adopted the model numbering system."
In collector 'verbal shorthand', the term Pre-Model came about as a way of differentiating between a gun that was stamped with a model # and one made before # stamping began, but was otherwise the identical gun.
So instead of having to say, for example: "I have a gun just like a Model 36 but it's not stamped Mod 36", it was shortened to: "I have a Pre-Mod 36".
That tells the listener immediately that it's not a Baby J. It has to be a Model of 1953 .38 Chiefs Special. If you used the term pre model for both a Baby J and a M1953, further conversation is then required to establish what's really being discussed or for sale!
Screw count 6, 5, 4, or 3 screw (depending on the frame size) is further used to distinguish between Pre Models. There is a Pre-Model 36 (Model of 1953) 4 screw and a 3 screw.
The actual definition of the true "Pre-model" collector term meaning should not be confused with the generic meaning of "pre".
In other words, in the generic sense any .38 cal. K frame S&W back to 1899 can be called a pre Model 10 but loses all the collector "Pre-model" term's benefits which are to describe revolvers that are exactly the same as numbered models but not yet stamped with a model number. Please click on the yellow '3' in the upper right of my name bar for a lot more detail.
The 'bright blue' finish did approximately correspond with the Model of 1953.