Claim: Betty Baskcomb only took live theatre roles in her entire career.
Evidence: Betty Violet Marie Baskcomb (30 May 1914 – 15 April 2003) was a British actress who appeared in roles for theatre, film, television and radio productions. 
Her screen debut was in the Hitchcock film The Man Who Knew Too Much (1934). 
She made her last television appearance in 1975, and died in 2003.