Probably:

A Mary S. McKay, aged 33 years (b. abt. 1888), travelled from Durham, South Africa to London, England. She was a passenger of the ship "Durham Castle" and arrived London on 11 June 1921. She was accompanied by Alex Hugh McKay, aged 47 years. Their permanent residence was in South Africa.

A Mary McKay, born 22 June 1887, travelled from Durban, South Africa to Glasgow, Scotland. She was a passenger of the ship "Media" and arrived Glasgow on 23 June 1956. Her address in Glasgow was 47, Park Hill Avenue at that time. She planned to stay 3.5 monthes.