First appearance = More Fun Comics #55 (May 1940)
<and reprinted in DC 100-Page Super Spectacular #6 (1971)>
It looks like Zor's other Golden Age appearance was in More Fun Comics #57 (July 1940), then he was resurrected (of sorts) in All-Star Squadron Annual #3 in 1984.
After that, Zor appeared in Grant Morrison's Seven Soldiers of Victory books a little over twenty years later.