These abbreviations represent the type of scene and the area where it's being filmed.
From this glossary and this page from abbreviations used in movie scripts,
EXT. => Exterior
INT. => Interior
O.S. => Abbreviation for Off Screen, denoting that the speaker is not resident within the scene.
V.O. => Abbreviation for Voice Over, denoting ...
In these three examples, you can find different ways of doing it.
It is denoted with "Spoken Title", "Sub-title" or just "Title" instead of the number of the shot/scene.
Here are examples from the links:
(1) The Sidewalks of New York (1923)
Scene 13--The battlers break and glare at
Reagan. One of them, a freckled specimen with
dangerous eyes, asks:
As a former screenwriter, I'm well aware of standard Hollywood story structure. Essentially, the story is broken into 8 sections of nearly-equal length, with sections 1 and 8 typically being a little shorter, and sections 2 and 7 typically being a little longer. Sections 3-6 comprise the "second act", where the main characters move past obstacles towards ...
There are an unknown amount of original scripts that have been bought by studios that have not been greenlit, only the studios themselves know how many.
The reason for them basically sitting on shelves collecting dust is that studios are investing in films that are fairly guaranteed to make them money, hence the prevalence of superhero movies and remakes ...