Glen was the only member of The Wrecking Crew who couldn’t read sheet music. That’s unheard of for a studio musician. Heck, their job description is pretty much being able to play sheets perfectly by the second take. Glen could listen to any composition once and instantly write a guitar part in his head then play it by ear. A real musical prodigy.
Not a studio musician, but Steve Morse doesn't read music either.
Most guitarists back then only read Tab (Tablature) (not to diminish his aural memory) and it's still that way today.
From the 1980s comedy 'French and Saunders', where Mark Knopfler, Lemmy Kilmeister, David Gilmour, Mark King, and Gary Moore give testimony in court that they can't play guitar/bass without Tab.
https://www.youtube.com/watch?v=QeIxJzdPD0A
For those avoiding YouTube:
https://dangerousminds.net/comments/revealed_david_gilmour_mark_knopfler_lemmy_cant_play_without_the_little