Many much older machines would have the diagram and the length, becasue the Internet wasn't invented yet and folks in sewing centers needed some guidance that might not be in their books.
Heck a few years ago I asked a Bernina dealer for a bobbin for a stell bodied machine. They looked in ther charrts and told me Bernina didn't make such a thing. They had alearning experince. but i still didn't get a bobbin.