An old recipe is a set of instructions to a competent cook, and it assumes a great deal. Quantities are approximate, timings are relative, and the reader is expected to know what the result should look like. There is no arithmetic beyond the ingredients.
Somewhere in the twentieth century a second kind of number appeared, printed beneath the method: a figure describing the finished dish rather than instructing the cook. That addition changed the genre.
Two audiences in one book
The moment a recipe carries a summary figure, the book is addressing two readers at once. One wants to cook the dish. The other wants to evaluate it before deciding whether to. These readers want different things from the same page, and the layout of mid-century cookbooks shows the strain.
Publishers solved it typographically, by pushing the figures into a small block at the foot or into an appendix. The compromise is still visible in recipe layout today.
Where the figures came from
Early recipe figures were derived by applying published composition tables to the ingredient list, dividing by the stated number of servings. That is a reasonable procedure and it is also a chain of approximations: table values, ingredient quantities and an editorial decision about servings.
None of that was hidden. Cookbooks of the period frequently explain the method in a preface, in a tone that assumes the reader will find it interesting rather than alarming.
What was lost and gained
What the figures gave was comparability across a book, which had never existed before. What they cost was some of the older recipe's confidence in its reader, who no longer had to be trusted to judge a dish by eye.
Both things happened at once, and the modern recipe is the result of that trade. Reading an older cookbook beside a newer one makes the exchange unusually easy to see.