At what address were the stairs in the TV show "Upstairs, Downstairs" (PBS, 1974-79)?

The stairs in the TV show “Upstairs, Downstairs” were at 165 Eaton Place in London.

The house actually filmed was No. 65, but to avoid lawsuits, the producers glued a “1” in front of the “65.”

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At what address were the stairs in the TV show “Upstairs, Downstairs” (PBS, 1974-79)?