I remember using #3 and #4 pencils in mechanical drawing classes in high school. They were progressively harder. I also see an occasional #2.5 pencil, just a little harder then we are used to!
Somewhere I have a drafting pencil that used a fat lead that was sharpened by using something like sandpaper.
Think the lead I used back in 1966 was probably a #1.
Still have a drafting set that belonged to my father-in-law. Most of my drafting tools were stolen along with a lot of my tools when my shop was broken into.