If memory serves,  I believe it was called APL\3000, notice the backslash as
opposed to the slash.  Also, you needed a special type of HP2641 terminal
which supported the APL character set.  In 1976-77 I used to develop
functions on a system supported by I.P. Sharp  (it think that is the right
name) in APL.  That is how I got some of my grey hair.  Later, on a Series
III with the APL board, I could stop a system in its computational tracks
with a single function, or even just starting APL.  I am glad those days are
long gone.
 
Kind regards,
 
Denys. . .