MIMF and the Guild of American Luthiers have many plans accurately drawn from vintage instruments (for example, I used the plans for Segovia's 37 Hauser when I built my classical). You can compare those to currently produced versions.
The change from "sloped" to "squared" shoulders primarily occured when Martin and Gibson changed from 12 frets clear to 14 - in most cases they didn't move the neck "up" but pushed the upper bout down to 14. Obviously that changes a lot - air box volume in particular - but in most cases it also means a model name change (000 to OM, slope shouldered or "S" model dread to standard dread).
Also, as you probably know, these guitars changed a lot over the years within a model name - sizes, shapes, bracing, sound hole location, scale length, bridge location..... might change for the same model from one year to the next. Here is one article that talks a little about that as well as the change from 12 fret to 14 clear.
FWIW, here is my short scale OM bodied 12 string and long scale 000 sixer - all built as close to 1930's shape as I could. The obvious sqared shoulder of the OM and bridge smack in the middle of the 000 are clues to their difference.
and lastly, here is the OM twelve compared to my very long scale copy of an old Stella - again, built from old plans. Check the fret spacing on that 26.2 inch scale neck LOL