You should check the actual manual: http://applelogic.org/files/AIIEREF.pdf

First, the cover uses italic sans-serif “I” characters, not forward slashes. It’s pretty clear if you’re familiar with typography.

Second, the body of the document uses neither. It uses the standard upright I character.

As for the boot ROM, well, at the time, the character set was a fixed bitmap, stylized text wasn’t a thing on PCs yet, and slashes were as close as they could get.