The "London Opera House" used here is actually the Wimbledon Theatre.
The film flopped on its release, and director Terence Fisher fell out of favor with "Hammer Studios", for whom he was not given another film until 1964.
The mask was made on the fly just before shooting out of cloth, tape, string and paint.
A subplot involving a pair of Scotland Yard police inspectors on the trail of the Phantom was shot especially for the American TV version (by the TV companies, not by "Hammer"). This was a regular occurrence in this era, most notably with "Hammer"'s films "The Kiss of the Vampire (1963)" and "The Evil of Frankenstein (1964)."