Amazing seeing this here. I hung out with him a few times back then, and I was just thinking about this again just a few days ago. It was really great spending some time with such a smart person. He showed me how he would write little notes and mind maps in the margins of books, and the peg system of mnemonics. One of the times I was there, he was teaching python lessons to someone. I tried to make sense of his notebook system, and to this day I still use color coding, and things like a triangle for pieces of data in my notes. Somewhere, I have or had a copy of this with a lot of writing in the margins trying to make some linear sense of it.

Lately, I've been keeping an "engineering" notebook, using similar technique to the original poster's technique: dated entries and a place for a table of contents (that I need to update).