I have not been able to find an actual version of the text of the Aero Games (Switzer) Thief.
I have seen people reliably relay that the class (a "Box Man") used spell slots for the abilities.
"It came about like this, one group had a dwarf who wanted to try picking locks with his dagger, so I had the idea for a Burglar class, which we drew up like a Magic user but with skills (like Lock picking) instead of spells. The consensus was to call the class “Thief”. According to D. Daniel Wagner.
I should note that in more recent times, Wagner has disputed the Switzer origin, and said that he invented the class and Switzer just reported it to Gygax.