The Top Key
The Top key first appeared on SAIL Keyboard around 1970.
It is like the Shift key, used to input Math Symbols ∞ .
Then it appeared on Lisp keyboards
Seems, after the Space Cadet, the Top key disappeared. But the concept is picked up, as Apple's ⌥ option key [see Mac Keyboard Viewer] and AltGraph key. [see Alt Graph Key, Compose Key, Dead Key]