From Evenings with Cary Grant:
His parents named him Archibald Alexander. Vicar EW Oakden baptized the child in the Episcopal faith on February 8, 1904, in the Horfield parish church. His baptismal certificate (which Grant said was lost in a Bristol fire during World War I) identified it as Alexander. Nonetheless, it was a child called Archie Leach who would become a man known as Cary Grant and achieve international fame.
Possibly because Grant himself had a lasting affection for his original appellation (he even named one of his dogs, a Sealyham terrier, Archie Leach), the public has long been aware that Cary Grant started out life as Archie. When he ad-libbed lines in His Girl Friday and Gunga Din referring to Archie Leach, they were inside jokes the audience understood. And when John Cleese played “Archie Leach” in A Fish Called Wanda, it was an homage to a beloved thespian.