Stanley S. A. Watkins (1888-1975) George R. Groves (1901-1976) The British electrical engineers who worked in America for Western Electric and Warner Bros. and played major roles in the development of 'Talking Pictures'.

see http://www.amps.net/media/files/library/journals/20_December_1996_1.pdf
"Brian Hickin of AMPS contacted Warners who undertook a thorough investigation. They reported that they’d sold their cinema chain, including their flagship Leicester Square cinema, to SBC International Cinemas in 2003, who’d re-branded them all as Vue. A re-development had taken place at the West End theatre, the plaque had been removed and it was never returned. No one knows where it is now, as Warners’ spokesman commented:
“Unfortunately, we are unable to come up with any clues as to its whereabouts. Even searching in the out of the way nooks and crannies of the cinema has not yielded results.”" - http://www.georgegroves.org.uk/blog_files/george_groves_news_2009.html

Su Lawson on Wikimedia Commons
Su Lawson on Wikimedia Commons
This is an approximate position

Plaque is now missing

Warners Cinema, Leicester Square, London
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by British Film Institute on 01 October 1996 #168 of the Centenary Of Cinema 1996 series

Colour: film cell

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See also...
Robert Pearce on Wikimedia Commons
Su Lawson on Wikimedia Commons