Who was billed as "?" in the opening credits of the 1931 version of Frankenstein?

William Henry Pratt (23 November 1887 – 2 February 1969), better known by his stage name Boris Karloff, was an English actor.

He was billed, in the opening credits of the movie that made him a star, as "?". Seventy minutes later, the end credits of James Whale's 1931 "Frankenstein" gave a name to the unrecognizable actor who had become the unnatural creation of Dr. F., and a fine monster-movie name it was: Boris Karloff. The mysterious Karloff was in fact a middle-aged, middlingly successful English character actor (born William Henry Pratt), skillful and professional enough to have appeared in dozens of pictures in the previous 15 years, yet stubbornly obscure: when he was suggested as a possibility for "Frankenstein," Whale is said to have responded, "Boris — who?"